ReAction is a replacement for the various action bars provided by the default Blizzard UI. With ReAction, you can group your action buttons into bars however you want, and have as many bars as you want. Those bars can then have a number of dynamic behaviors assigned to them. ReAction provides an intuitive mouse-driven interface for arranging and resizing bars, and a dialog interface for the majority of the configuration options.
There are many action bar addons out there. I suggest you try several and choose the one that works best for you. Other popular action bar addons include:
Note that action bar addons are by nature generally incompatible with each other. Only install and enable one at a time! Most can currently be found on WoWInterface.com or Curse Gaming.
The WoW AddOn authoring community is rich and diverse. Virtually every successful addon author learns by reading, dissecting, and even pilfering (:-P) others' code. ReAction owes its existence to the following:
ReAction embeds components of the Ace2 framework.
The design goal of ReAction is to be user-friendly and feature-efficient. Useful features are incorporated: "fluff" is culled out. Many behaviors have been inspired by other addons.
None of these limitations are fundamental (except perhaps the keybindings). With the extensible architecture, an enterprising mod author could certainly write an addon module that extends ReAction's capabilities.
ReAction is distributed as a ZIP file: like all addons, just extract the archive contents into World of Warcraft/Interface/Addons. This will create a new folder called 'ReAction' in the Addons folder. Restart WoW, check that the AddOn is enabled in the character-select screen, and away you go.
A common mistake is to extract the contents into a new folder named ReAction (thereby creating a doubly nested Addons/ReAction/ReAction), or to rename the ReAction folder. Make sure that the path to this README file is World of Warcraft/Interface/Addons/ReAction/README.html.
Dynamic bar behavior is a complex topic. At its most basic, you can have a bar switch between pages if you change stances/forms/stealth, just like the Blizzard UI does. For the more advanced user, multiple states can be created and transitions defined based on the available events as dictated by Blizzard's secure button API.