What are friends?

Definition of friends
a person with whom one has a bond of mutual affection, typically one exclusive of sexual or family relations.

I think outside of dictionary meaning, friends are simply people that you happen to know and like to a certain extent.
Friends are not the same as siblings or anyone with blood relationship to you. Friends are also not romantic in nature. An ex-husband and ex-wife could never be friends.

Friends are outsiders that you happen to know very well. A friend is basically converted from stranger.

Me, my brother and my sister can never be friends. Cos we are siblings. Sibling is bounded by blood ties naturally. Whereas friendship is forged through time, not by birth.