From Diablo Wiki
Tool tips, also known as "hover text" are the pop up boxes of text that display when you hold the cursor over an item, skill icon, interact element, or anything else in a video game. These tool tips display additional information about the object you are pointing at, usually explaining the function or adding color or detail.
While many games have very poorly-written tool tips that were clearly thrown together at the last minute, the Diablo III developers (in typical Blizzard well-polished style) have put a lot of time and effort into the text boxes. The aim is to describe and inform, with as few clearly-purposed words as possible.
Tool Tips During Development
Tool tips were in Diablo III all during development, and fans saw them in the playable demos at each year's Blizzcon. Those tips were always works in progress though -- many of them referred to removed skills, old names, or changed features. This is not a surprise; in-game documentation always lags well behind the actual game changes since there's no point in constantly updating the tool tips when whole features are being changed around in every patch.
Not until they were preparing for the beta test and many of the game features were semi-finalized did the developers put in some serious effort to improve the tool tips. From the lack of complaints about them during the early portions of the beta test, it seemed that most fans were okay with how the hover text was presented. As the beta progressed though, code diggers and stat geek type fans started to notice some problems.
For instance, damage over time was calculated differently for characters who were dual wielding, and the tool tip was misleading in that instance. A forum thread devoted to the issue]  sprung up, and earned a December 3, 2011 reply from Bashiok in which he explained the game feature, and gave some insight into the D3 Team's thought process regarding tool tips. 
- Even though we say continuously, there’s a pulse rate under the hood, so do players want to know this pulse rate?
- If we did show a pulse rate, we’re adding more complexity to an already complicated tool tip.
However, the benefit of being able to directly compare 135% weapon damage on Disintegrate to X% weapon damage on another skill is super high, so we love that.Stepping back and going big picture, we’re currently working on greatly simplifying tool tips. It’s too early to give details at this point, but our general feeling is that we provide way too much information in the skill tool tips in general.
Fans didn't know what to think about that, as no one seemed to find the current tool tips that complicated. This issue was soon returned to, with much more impact.
Tool Tips Simplified
In mid-December 2011, during the Diablo 3 beta test, Blizzard announced a global initiative to simplify and streamline the tool tips in Diablo III. As no one playing the Diablo 3 Beta seemed to think the tool tips were unduly long or detailed, this announcement was something of a surprise to most players. 
Take a look at the skill information on the Diablo II Arreat Summit for [http://classic.battle.net/diablo2exp/skills/barbarian-combatskills.shtml barbarian combat skills]. We think that is a great amount of insight into the mechanics of the skills, and it’s a great example of what we want to make available for you here on the Diablo III website. We all have to agree though that it’s a ridiculous amount of information to try to display in-game, especially through a tool tip. Let’s be clear though that our intent isn’t to force anyone to alt-tab to get game information. The tool tip information we’ll be providing, as well as very basic concepts of players getting and equipping more powerful items, will be plenty to get all players through the game, at least on Normal.
Some of you have been discussing tool tip complexity and more than a few times the World of Warcraft solution of having two different sets of tool tips (simplified and normal) has of course not escaped us. It’s a solution we’d like to explore, but if we did it’s extremely likely that our “normal” tool tips would be the toned-down versions we’re working toward now, and the “simplified” would be even more basic, giving game intent advice in place of any numbers. It’s something we’d like to explore but it’s still something we’ve yet to design, and could very well be something we don’t end up getting into the game.In closing we wanted to give everyone a heads up because we know there are going to be some conclusions about our intents, and we wanted to share some ideas of having something similar to what Diablo II players could find on the Arreat Summit back in the day.
The in-game reference guide/book idea for skill and game information has a number of UI issues, but as a player of the game it’s not a great solution because:
- You’re not always in-game. More often than not when you’re dissecting a skill at a massive level of detail you’re on a website/forum discussing things, or you’ve got an excel spreadsheet open. Or you’re getting in some extra Diablo time while at a friend’s place. Or you’re at work on your lunch break.
- A web page lets us present far more organized detail than an in-game book would allow (without ceasing to look like a web browser embedded in a book which would just look silly). You want tick rates, scaling formulas, proc interactoins, etc.
- A web site allows us to provide the most timely information. If questions come up on the forums, we make a hotfix, we find errors, or come across more information we’d like to pass along, we can get that information out to players immediately. If it was in-game it would require a patch.
Aside from "accessibility" jokes about how Blizzard had even "nerfed tool tips," most fans were supportive of this change, or at least not especially opposed to it. Hardcore players were all planning to read wikis and other guides for the full detailed info, including stats and charts that would never have been included in Diablo III itself, so the in-game info wasn't a real concern to them.
The finalized, "simplified" tool tips were unveiled in Beta Patch 10 released on 23 January 2012. Below is a selection of skill tool tips comparing the simplified with the full skill description.
To activate the full skill tooltip, you can press CTRL while hovering over the skill.
To keep everyone satisfied, Blizzard added an option to permanently enable advanced tooltips in Beta Patch 13. This setting can be enabled in the gameplay options window.
More Tool Tips Added
During the beta test, players noted numerous added tool tips, for some of the most simple and basic features in the game. Messages now warn new players of the most basic game actions, such as damage taken, life healed, item picked up, stash accessed, new skill earned, etc.
- November 5, 2011: Upon Closer Inspection #8: Numerous tool tips added.
- November 23, 2011: Upon Closer Inspection #10: More new tool tips screenshots.
A few of them are shown in screenshots below.
- Confusing Tool Tips - Diablo: IncGamers 14/11/11
- Simplifying Tool Tips - Blizzard 3/12/11
- Simplified Skill Tool Tips - Blizzard, 8/12/11
- Tool Tip Parady - Azzure, 12/12/11
- In Defence of Simplified Tool Tips - Blizzard, 13/12/11