Alignment reflects how your character reacts to the concepts of good, evil, law and chaos.

Bastions of War

On BoW, class alignment restrictions have been removed. Normally, only good characters could be Paladins or evil characters Assassins; but, since teams are based on alignment, any alignment can be any class. This applies to the law-chaos axis as well; Bards can be lawful.

Why can't I switch my alignment to join the other team?

This isn't some game, soldier! This is war -- the ancient battle of good vs. evil! You're expected to support your brethren and destroy your enemies! You can, however, change your alignment once before you reach the starting level (just see Shady Character).