Advanced 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 is because your choice will not only make your character more distinct and powerful, but it will also help further define the role you wish to play in The Old Republic.
In addition to your Class-specific Skills and Abilities, your chosen Advanced Class will grant you access to role-defining Abilities that represent additional areas of expertise and some may even grant new weapon and armor types. Additionally, each Advanced Class will also have three sets of Skills available to further customize your character.
http://www.swtor.com/sites/all/files/en/gamesystems/advanced_classes/skills.jpg)
The chart above is not representative of any of the current Advanced Class Skill Sets, nor does it show exactly how many Skills you may potentially access. It is included to provide an example of how a character, in this case a Sith Warrior, may look as he progresses through one Advanced Class (note that the other Advanced Class Skill Sets are disabled and cannot be trained).
As mentioned above, each Advanced Class opens up three Skill Sets with a variety of Skills in each. Unlike selecting an Advanced Class, these Skill Sets are less restrictive and your character can obtain Skills from any set or from all of them!
Source (
http://www.swtor.com/info/systems/advanced-classes)