Character creation Seasonal check box.
Ladders are part of Seasons and after being long-promised were added to Diablo 3 in Patch 2.1 in the summer of 2014. Full details on Seasons are not yet available, but Blizzard has promised they will reset regularly and feature a large amount of unique content, including new Legendary items, plus special Seasonal Leaderboards ranking the top players by numerous criteria. There will also be special seasonal Achievements called Conquests where only the first 1000 players to complete them, each ladder season, will be publicly honored.
Seasons will run for several months (quarterly?) and are optional and require an opt in at the time of character creation. For the duration of each season, your ladder characters are stored separately on your account, and do not share gold, stash space, Artisan levels, Paragon Levels, equipment, etc with non-ladder characters. After each season ends, Ladder characters are merged back in with your regular characters, and any gold and Paragon experience earned by your Ladder characters will be added to your main account.
It's not yet known how end of season character merging will be handled in terms of players exceeding their total stash space, and Blizzard has not said if additional character slots will be provided for ladder characters.
 Ladders are "Seasons"
The official name for ladders in Diablo 3 is "Seasons" which is a bit confusing, but apparently meant to set them apart from the much simpler Diablo 2 ladder system.
Similar to Ladders in Diablo® II, Seasons will offer players in Diablo® III a chance to periodically start fresh, leveling their level 1 characters and artisans from scratch. Season participation benefits include exclusive rewards and unique Legendaries as well as the the thrill of climbing all-new DiabloWikiLeaderboards by completing achievements, earning Conquests, or running Greater Rifts.
 Ladder Display
Leaderboard first sample image.
Ladder positions during each Season will be displayed via leaderboards. The first sample image of one was provided in the Patch 2.1 preview, with more info to come.
Fans have been asking about ladders since Diablo 3 was first announced back in 2008, and while the developers always sounded open to including them, it took years before the idea moved beyond the drawing board. Here's a chronological collection of official comments on the idea of Ladders in Diablo 3.
Early comments on ladders were of the "maybe someday" variety. Community Manager Bashiok talked up other game features as a better way to rank players in September 2010. 
As far as the e-peen meter goes for showing off a higher level character, we think a proper achievement system is much better at this. There’s more variety, they’re more interesting and compelling for people to work toward. One important point of note is that our achievements are cross-character, meaning we can give achievements for things like leveling 5, 10… hell, 20 characters to 60. Certainly a bigger feat than a level 99, and we can give tangible rewards for it too.
Jay Wilson talked about wanting ladders someday, but not any time soon in August 2012, in relation to the new Paragon system.
Could we see a Ladder system get tied into the new Paragon system?
Currently no. We don't have any plans to do a ladder system. Probably the question for that is "are we going to have some kind of seasons for the game like Diablo 2 did". I said in the past we are not really interested in doing that, but I really should clarify that. The idea of doing seasons is a really good idea and something we should introduce to Diablo 3 at some point. I don’t see no reasons for it to be at launch, as that is Season 1. We don't have to worry about it till we do Season 2. What im not really keen on is leveling being the primary element that motivates doing a new season. I don’t think it appeals to that many people.
The people that like it really love it and they compete. But everybody knows it does not take that long to get to level 60 in Diablo 3, and it did not take that long to get to level 99 when you level raced in D2. The people that would do that race, they would do it for a couple weeks, and then essentially the season would be over, and the ladder would be over, and that is something we do not want. We want something that feels like as if we introduced seasons, We want whatever the motivation to join the new season be to be more appealing to a broad group of people. That’s not to say we don’t want it to be appealing to our more hardcore dedicated players. We want it to be super appealing to them as well. We just think there are other things than “lets all just level up a level ladder”. So totally for Seasons, not really that keen on the level ladders because I just don't think a big part of the audience benefits from it. If you're basically not in the top 100, who cares?
That means that basically 100 people think it’s awesome, and maybe a couple more hundred think its cool because they think one day they will be in it and to me it’s like, ok really? Like, we are going to put a big feature in, and I’m not saying those people are wrong, or that I don't want to make something for them, I just think we can do better than that.
14 March 2014
A series of cryptic messages and images appear on Twitter from Josh Mosqueira. 
We only speak of it in hushed tones… we didn’t just build walls out of bones, but also ladders to climb over them…
Ladders at the #reaperofsouls launch event?!?!?
|Devs pose in the warehouse
14 September 2013
Travis Day speaking on Paragon experience being reset with new seasons, 
Paragon experience will be account but will also be broken out by game play mode. Players who play in Hardcore will have a separate hardcore paragon level from their non hardcore characters.
When a season ends all of the paragon experience you have earned will be rolled into the appropriate pool of paragon experience for the account and you will be able to start at 0 again in the new season.
This is all still a work in progress and subject to change but that is what we are currently intending.
Selecting "seasonal" on character creation.
13 June 2013
Blizzard community manager asks for input from gamers on what they would like to see in a ladder system in Diablo 3. 
- How would you ideally like that system to work?
- How long would you like the resets to be?
- How many characters do you think players should be able to create for ladders? (Do you think they should take up the existing slots you have?)
- Should there be an auction house for ladder characters?
- Should there be any special items or perks for ladder-only characters? If so, what kind?
- What kind of restrictions, if any, should be placed on ladder characters?
- Do think players should be able to bring their ladder characters into “non-ladder” once a reset is complete?
- Is it necessary for ladders to be competitive, or is it enough for players to just see for themselves how far they can go on their own?
- Why would ladders improve your in-game experience?
15 May 2013
Wyatt Cheng in a video stream back in May of 2013 addresses the issue of how a ladder resets the economy. 
There are two aspect of ladder, both of which I think are interesting. One is the competitive ladder aspect, the race to the top. The other is the economy reset, this fresh sharded off economy where everyone begins clean. ...(snip)... So I think one of the benefits of a ladder is just this idea that it's really fun to start fresh, and experience this rapid rate of upgrading, again. A ladder is a great excuse to do it. Sometimes we all want to do it and we can choose to do it, but sometimes it's nice to do it together, as a global community. Or have a recognized badge of honor, for having done it.
And later what excites him about ladders and how it could not happen until and expansion.
We're definitely looking into the possibility of a ladder, or something like that. I personally -- this is just me speaking and there are other people on the team who would disagree with me, so I'll say that, it's just my opinion -- I'm more excited about the reroll and clean economy aspect of ladder seasons, than I am of the race. I know the race is exciting too, and there's pictures of people who are getting ready for the new season. They have all their caffeinated drinks on their desk, and I think that's epic and awesome, if not always healthy. But realistically, yeah it's something we're cool with. It wouldn't be viable until an expansion.
Travis Day also chimed in with enthusiasm but still, stresses the fact they will happen later rather than sooner.
Ladder is something that is not out of the question. We talk about it, we're investigating it, you might see it one day. Not making any promises on when or how, but it is certainly something that we talk about a lot. A lot of the talk we do, or sometimes do, is like -- Diablo needs to live on well beyond the point of which we're working on the game. D2 had some support, but you're not going to see (and I'm pulling time frames out of butt, but): Five years from now, you're probably not going to see us making huge content patches, or class updates. At some point we'll set the game aside, sort of leave it as a work that we've completed, and move on to like Diablo 4, or whatever, right? Ladders is absolutely one of the things that helps the game stay fresh for people 5 years from now, or 3 years, or 2 years, or whatever. Ladders are cool, ladders comes with a lot of baggage when I say "ladders"; but the idea is of sort of a fresh economy and a fresh system for people who choose so, to opt into leveling all over again with a clean slate, with other people who want the same thing. That's really important and it's really compelling, and we probably want to have that one day, but I don't know how we'll implement it, or what it'll look like, or when it'll go in.
Ladders and seasons were first confirmed as a feature we would definitely see in the future by game director Josh Mosquera in March 2014. 
... we’re still working on them, but the general gist is that you’ll be able to level up a character, we will have specific ladders for seasons, and also some goodies that will drop along the way. There’s more information of that to come, and we’re not making an announcement of when, but for sure Seasons are coming to Reaper of Souls. Yeah, the secret is out.
25 March 2014
Josh Mosqueira unequivocally confirmed details of ladders coming to Reaper of Souls in content Patch 2.1. 
Well that's a really good question, isn't it, Wyatt? When you're making a game you come up with a lot of good ideas, and some ideas like Devil's Hand you say: it's awesome, it probably needs to bake a little bit longer. But one of them that we felt was really important, that we felt we really needed to take the proper time to develop it, was one of the features you guys have asked about a lot. You might have noticed that Wyatt and myself have done a lot of research for something called ladders. I'm very happy to say that as part of our first big content patch coming out in the next couple of months, we are going to be releasing Seasons, which will include a ladder-like aspect to them.
Well, we're still working on them, but the general gist is that you'll be able to level up a character, we will have specific ladders for seasons, and also some goodies that will drop along the way. There's more information of that to come, and we're not making an announcement of when, but for sure Seasons are coming to Reaper of Souls. Yeah, the secret is out.
In an interview in April 2014 John Hight fleshed out more concrete details of how Ladders and Seasons were shaping up during development and what players can expect to see. 
- Each Season begins with a fresh characters
- No access to existing stash, loot or shards
- When season ends all experience, items, gold etc is rolled back into your normal non-ladder roster
- New, powerful legendaries that drop only to ladder characters
- Leaderboards will rank progress by various metrics such as region, clans
24 April 2014
John Hight in an interview gave up some more details on what's currently in ladders and seasons.
Seasons are a shard of the existing game where everything is a fresh start. You start with a brand new character without access to any of the gold or items you collected with your primary account for the length of the season. We plan on creating brand new, powerful legendaries that only drop within in the season. After the season ends, all of your progress — your experience, your items, your gold, etc. — will be rolled into the main roster. The goal is the fresh-start feeling — so many changes and tweaks have happened since the launch of Diablo III — it is really energizing and fun to recreate that ‘first-moment-in-a-new-game’ feeling.
We’ve also added leaderboards. Players can race on a variety of metrics to try to become the leader in their region or the top of their friends and clan lists. That friendly competition angle lends itself really well to Diablo games.
- ↑ Achievements better than ladders - Blizzard Community Manager, 23/09/2010
- ↑ Ladders not coming soon - Jay Wilson interview, 20/08/2012
- ↑ Cryptic latter tweet - Josh Mosqueria, 14/03/2014
- ↑ Resetting Paragon points - Travis Day, Blizzard Entertainment, 14/09/2013
- ↑ Player input on ladders - Community manager, Blizzard Entertainment, 13/06/2014
- ↑ Economy reset - Wyatt Cheng, Blizzard Entertainment, 15/05/2013
- ↑ Ladders coming to Diablo 3 - Josh Mosqueira, Blizzard Entertainment, 25/04/2014
- ↑ Ladders coming to Diablo 3 - Josh Mosqueira, 25/03/2014
- ↑ 9.0 9.1 Details of Seasons Shacknews interview with John Hight, 24/04/2014