A friend is someone who should always have your back, no matter what. A friend can be one with few similar interests or many, but they never beat you down because of what you may be or what you may believe in. They're someone you can rely on, someone you can be honest with, someone you can laugh with and someone who can be your shoulder to cry on without being afraid of them ever turning you away. They're always there, maybe not physically, but in one's heart.
A bit long and a bit deep but yeah, that's what a friend is to me.