There are many different kinds of friendships, but at bare minimum, I would consider someone a friend if we both desired to and voluntarily talked/hung out with each other, for no other reason than each other's company. I would say this is sufficiently broad enough to encompass a wide variety of friendships, but also carries a degree of exclusivity, so that not every person you talk to is your friend. Even if you don't actually have time to be around them that often, it's the thought that counts. Not just 'cause you're bored and you can stand this person, but because you genuinely find them interesting and would generally rather spend time with them than not (I say "generally" because alone time is important for pretty much everyone, too). I think that's the root of friendship. Everything else, as far as I'm concerned, is a bonus.
Based on this definition, most of the people I know I would classify as acquaintances, with very few yet highly treasured "real" friends.