From Diablo Wiki
Seasons are Diablo 3's version of the Diablo 2 Ladders, and they were added to Reaper of Souls in Patch 2.1 in August 2014. When a season is active, players can make new characters (hardcore or softcore) and click the "Seasonal Hero" button during creation. That will designate their characters as Seasonal, and they will only be able to play or interact with other Seasonal Characters for the duration of that season.
Each Season is a fresh start; players can not bring in any of their existing characters, items, paragon points, gold, artisan levels, etc, from their normal account. This puts everyone on a level playing field, and Blizzard sees it as a way to bring players back into the game, as they can play with others of their power level, rather than trying to join public games with Paragon 700 Torment 6 steamrollers.
Seasons will run for a TBD time, presumably around 3 months, and when a season ends all gold, items, characters, etc are merged back into a player's main, non-seasonal account. As each season ends a new season will begin, and if players want to take part they must create new seasonal characters and go from scratch all over again.
 Leaderboards and Conquests
During each season, top players are ranked on the Leaderboards by various criteria.
- Greater Rifts: Leaderboards will track the highest level Grifts (and the time to complete) for individual players by class, as well as parties of 2-4 characters.
- These leaderboards can be sorted server-wide, clan-wide, just by your friends list, or by your physical location, such as at a LAN party or college dorm.
- Conquests: These seasonal-achievements record the first 1000 accounts to complete them on each server.
- First to level 70. The first 1000 of each class to level 70 are recorded on a Leaderboard.
- No max Paragon Level leaderboard. Blizzard specifically did not include a highest Paragon Level leaderboard since they didn't want to incentivize mindless experience grinding.
- See also Seasonal Legendary Items
 When a Season Ends
Players had many questions about how a Season's ending would be handled, in terms of merging experience, gold, items, paragon points, artisan levels/recipes, and more... from the seasonal account back to the main account. Blizzard ran several trial seasons during the PTR testing of Patch 2.1 in August 2014, using that to iron out the kinks in the system.
A community manager answered numerous fan questions during that time:
After PTR Season 1 has ended, the Season roll-over will begin. During the roll-over, your Seasonal heroes, inventory, shared stash, and Paragon experience will be transferred to your non-Seasonal profile.
Normal heroes, inventory, shared stash, and Paragon experience will transfer to your Normal non-Seasonal profile. Similarly, Hardcore heroes, inventory, shared stash, and Paragon experience will transfer to your Hardcore non-Seasonal profile.
- Heroes: At the end of a Season, your Seasonal heroes will become non-Seasonal heroes. They will appear in your normal roster. Any items, equipped or in the hero's inventory, will roll-over with the hero itself.
- Shared Stash: Items in your Seasonal heroes' shared stash will be mailed to your Normal/Hardcore non-Seasonal profile. These items can be retrieved by entering a game on any non-Seasonal hero and clicking on the mail icon located in the lower left of your screen.
- Paragon Experience: Paragon experience earned during a Season will transfer over directly without any additional involvement required from players. All Paragon experience will transfer, with experience gained by Normal Seasonal heroes going to the account's Normal non-Seasonal profile, and experience gained by Hardcore Seasonal characters going to the account's Hardcore non-Seasonal profile. (Note: Non-Paragon experience will not transfer.)
If you experience any issues with the roll-over, please let us know in this thread, and provide as many details as possible. The more information we have, the better we'll be able to identify and address the core problem.
Please, can you explain what is going to happen if we already have all characters slots full in the normal roster? The seasonal character its going to be added in a new character slot?
In order to create a Seasonal hero, you'll need to have an empty character slot in your roster. If you do not have an empty character slot at the start of a Season, you will need to first delete one of your active heroes.
Will achievements earned in season mode also be rolled over to non-season and granted there (if you didnt previously have them in non-season)?
Under current design, if you unlock an achievement on a Seasonal hero that you haven't already unlocked on a non-Seasonal hero, you'll gain credit for both at the same time.
1. Does gold count as "inventory" and will also be transferred?
1. Gold will be automatically transferred from your Seasonal profile to your non-Seasonal profile. Ditto for Blood Shards.
2. Other progress such as artisan levels or crafting plans your artisans have learned will not be transferred/refunded in any way, right?
2. If your Seasonal Artisan is higher level, its level should transfer at the end of the season. Any recipes you have on your Seasonal Artisan that are not on your non-Season Artisan are transferred as well.
3. I second one of the questions above - what happens if your normal roster stash doesn't have enough space for all the items in your seasonal stash?
3. All items within the Seasonal stash will be mailed to your non-Seasonal profile at the end of the season. These items aren't automatically placed in your non-Seasonal stash, so you can wait and make room first before claiming them (if needed). Under current design, mailed items from your Seasonal stash will remain available for 30 days after a Season has ended.
Note: Seasonal heroes will be separated by mode, i.e. Normal and Hardcore. Normal Seasonal heroes will become Normal non-Seasonal heroes, and Hardcore Seasonal heroes will become Hardcore non-Seasonal heroes. Paragon experience, Artisan levels, items, gold, etc earned by Seasonal heroes will transfer to the appropriate non-Seasonal mode.
I believe (correct me if I'm wrong) that a seasonal character already takes up one of the slots and thus it will simply stay in that slot, but have its seasonal tag removed.
Quick note about Blood Shards and Season roll-overs:
When transferring over your Blood Shards from your Seasonal profile to your non-Seasonal profile, we will temporarily ignore the 500 cap.
For example, let's say my Seasonal profile has 200 Blood Shards and my non-Seasonal profile has 400. At the end of a Season, the 200 shards from my Seasonal profile will combine with the stack of 400 from my non-Seasonal profile, giving me 600 shards total (not 500, the normal cap). This is deliberate.
The caveat here is that players will be unable to pick up any more Blood Shards until they drop below 500.
What will happen to legendary gems? Will their levels add to those of the same non-seasonal legendary gems? Will you have two of such gems? Or will it simply end up being the higher level gem becomes the non-seasonal gem?
Under current design, you'll have two of such gems.
wat about non-season can we get item wipe and ladder reset there are still 2 sockets weapon in game...and people use The Furnace for ladder etc.
We will be performing a PTR-wide wipe for all Leaderboards this evening as well. This wipe will be global (i.e. it'll affect Seasonal and non-Seasonal Leaderboards).
what if it is the same level and the seasonal artisans have different recipes or new recipes, will it transfer? because you kinda linked to if it is only higher level.
 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.
 Blizzard Comments on Ladders in Diablo 3
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.
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
We only speak of it in hushed tones… we didn’t just build walls out of bones, but also ladders to climb over them…
andLadders at the #reaperofsouls launch event?!?!?
|Devs pose in the warehouse||Josh Mosqueira|
14 September 2013
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.
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
And later what excites him about ladders and how it could not happen until and expansion.
Travis Day also chimed in with enthusiasm but still, stresses the fact they will happen later rather than sooner.
25 March 2014
- 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
- ↑ 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