Friends are people with similar interests as you.
The friends I have are what they are because we grew up together, from early middle school and past highschool.
I don't actually physically meet with them much anymore though, but we are still great friends. We went through the runescape and WoW mmo phases, talk about our interests like computers or cars, stuff like that.
It's when you can just talk about anything, and you don't have inhibitions about it. When you talk with a random person, you wont talk about your feelings, life situation, or stuff you masturbate to right? With friends all that stuff is open and you can just talk about it without feeling nervous.
I don't really know how to make friends though, I've had these very few same friends since forever. I didn't have friends before them, I moved around a lot.
Is it worth it? I don't know, sometimes its a bit annoying when they want to talk and you don't.