This round of closed beta testing is expected to have 20,000 users, and begin work on quite a bit of new content. Six tribes are currently available, eight different classes, and 88 character models. The new content introduced with CBT3 includes new maps and 68 new techniques. Some new instanced dungeons and a party invite bulletin feature will be added as well, along with generally expected features such as in-game voice chat (provided by Hangame) and the ability to swim, jump, and climb. Look for the addition of Quick Menu and equipment items as well.
You can take a closer look at TERA at the official site, and check out the gallery of screenshots and concept art just below for a visual overview of the game.