Each class has 2 advance class specializations. To avoid confusion these are like subclasses within classes you get to choose at level 10.
So you:
1 - Choose a class to create your character.
2 - At level 10 you can choose one of two advance classes.
3 - In addition to your advance class selections, you can further assign talent points to further define your role.
Example:
1 - Class: Jedi Consular
2 - Advanced Class Chicoes: Jedi Consular Sage or Jedi Consular Shadow
3 - I choose Jedi Consular Sage. Now I can assign talent points to further enhance either my DPS capabilities as a Sage or my Healing capabilities as a Sage.
Hope that makes sense. Here's a link to Advanced Classes:
http://www.swtor.com/info/systems/advanced-classes