Dark side points
Dark side points are points you earn by making decisions throughout the game. Most of the time these decisions are bad, or what a Sith would do; but in some cases dark side points are considered just. For instance, if you are in a village and they ask you to help them with a threat while your superiors have commanded you to go away. If you help these villagers you may gain dark side points even if it was the "right" thing to do.
[edit] Effects on characters
The more dark side decisions a character makes, the more their appearance will change to reflect this. Their armor will have a darker look, facial features such as glowing eyes and purple veins start to appear, and the NPCs in the game will respect you in a different way.
Dark side points are permanant, but if a character wishes to switch to the light side they simply need to collect enough Light side points to unbalance their alingment.