In order to get useful and realistic feedback from real users, some software companies deliver pre-release software (a beta or beta trial) to a select number of users to try it out and report bugs, make suggestions, or just give significant customers clear evidence of the state of the product and dispel rumors of vaporware.
What is a beta? Edit
In terms of the software development, beta is usually the stage where all major feature implementation should be finished and the software is in the phase of bug fixing, refinement and stabilization before declaring a Final or Release candidate (sometimes called "Gold" as well). Sometimes major changes occur in this stage, but usually only in response to feedback during a beta trial.
Beta, of course, comes after the Alpha stage where major feature implementation usually occurs, is known to be unstable, and not generally fit for anything more than internal trial of carefully selected users. The stage before Alpha some call "Pre-Alpha", "Proof of Concept", or "Prototype".
These stages are semi-formal within the software industry, so "your results may vary".
World of Warcraft Edit
Before the official retail release of World of Warcraft (i.e. before you could buy the regular or Collector's Edition box in the store), there was a long closed beta, a short Beta Stress Test, and a short open beta. During this time, these lucky people got to play the unfinished version (client and server) at various stages as it approached the state Blizzard deemed good enough for release.
The Burning Crusade EditThis section concerns content exclusive to The Burning Crusade.
Wrath of the Lich King EditThis section concerns content exclusive to Wrath of the Lich King.
Opt-ins available for Wrath of the Lich King beta Blizzard has announced that opt-ins for the beta test for World of Warcraft: Wrath of the Lich King are now available in the US and Europe. To enter, click here for US accounts or here for EU accounts. The Wrath of the Lich King beta is open.
Beta keys were given out to BlizzCon 2007 and Blizzard Worldwide Invitational 2008 attendees and Blizzard also gives out Beta keys to various organizations (fansites, etc.) for promotional reasons.