A 'nigga' is a black person, and can be used as an insult OR a nick-name. Some people use the term 'nigga' as friend, and do not mean it to be offensive, however nowadays many people take it the wrong way.
1. Sam: Hey you, you nigga!
Sam: Yeh, you.
Tom: Don't call me a nigga! Just because
I'm black doesn't mean I should be called