http://www.swtor.com/info/systems/advanced-classesAdvanced Classes are specialized careers that your characters can choose in Star Wars™: The Old Republic™. As your character becomes more seasoned through adventure, you will be given the opportunity to undertake one of two Advanced Classes. This decision is equally as important as when you originally created your character. This decision is equally as important as choosing your initial class at character creation. Your choice will not only make your character more distinct and powerful, but will also help further define the role you wish to play in Star Wars: The Old Republic.
Your choice of Advanced Class not only defines which skill trees are available to your character (there are two unique trees per Advanced Class as well as one shared tree for the class), they also provide access to various special abilities, such as the ability to dual wield Lightsabers, wear heavy armor, etc.
These skill trees are not finalized and are subject to change between now and launch.