Friend:
QUOTE
A person whom one knows, likes, and trusts.
You know them, ie, you've met.
You like them, ie, you enjoy being around them.
You trust them, ie, you have faith they won't steal from you.
They're for various things, but in the end you both offer a service to eachother. Be it that you make them laugh, you're attractive, you stick with them when they need you, doesn't matter. In laymen's terms, friends are for helping one another.
I know people who try to be friends with everybody, and it seems to make them fairly happy. I can't do that, because there are some people that just annoy me too much. However, when I'm new to a place, and I don't know anyone, I'm pretty leinent of who I become friends with. When I have many, I don't engage people's friendship often. That being in a new place and not knowing anyone could be as short as an hour and not even very far away. For example, on Diablo II, at first I didn't know anyone, but it wasn't long before I became friends with a couple people. Even though I've eventually found some these people to be really interesting and funny, the engagment of friendship was based on a need to be social. If you've ever played Diablo II alone, you know what I mean...
When I'm with a bunch of friends, the only reason I become friends with another person is because they either make me laugh, they're one of my friend's friends or they offer a unique service. Not to say I don't have to offer a service as well, because if you havn't noticed, I'm only talking about mutual friendships here. I find, a lot of the friends I've made have been because they thought I was funny, and wanted to hang out with me. Since people who understand my jokes tend to be witty, we often end up friends because we make eachother laugh. But I'm also a rather patient and helpful fellow as well, and don't shy from offering to spend my time helping them fight bordom or a problem even when it's out of my way, for a friend.
I find if I have no desire to become someone's friend, I don't go out of my way nearly as much for them (so as not to give the wrong idea).
Sorry for the long answer, but you asked a rather broad question.