My understanding is that the anti-hero is a protagonist with the opposite attributes given to the classic hero (brave, idealistic, good, etc.) but the anti-villain is an antagonist with the opposite attributes given to the classic villain (evil, cowardly, destructive, etc.).
People gave good examples of each from pop culture such as Batman, The Good (from The Good, the Bad, and the Ugly), and on as anti-heroes... and the likes of Ozymandias or Saren as great examples of anti-villains.
In your particular case, your Light-Side Sith would either be an anti-hero or a hero because for you, your character is the protagonist of the story. She has the ideals and morals of a good person, and depending on the way you take her story, she could very well just be a hero who overcomes the adversity of the culture she was born to... or well, be an anti-hero because she has strong morals but still works for an organization as bad as the Empire.
She'd only be an anti-villain if she was an antagonist - that is, the story isn't told from her perspective, she was there stopping the hero (anti- or not) along the way, even if for good intentions.
Apologies if someone made this distinction already, but I admittedly didn't read every post thoroughly.
... I love you?
I am so going to show this to Krissie; you make very good points here, and I am hard-pressed to agree with them!
I find that in the whole shape of things (putting all the classes in perspective of the unfolding of the entire game) my Sith would be the anti-hero in that she's part of an Empire trying to take over, but she's still a hero of sorts because she is compassionate and trying to change said Empire from within.