I've known my best friend since we were both 13, so over half my life. The background for our friendship is rather special - because we both went to different primary schools, and we were both bullied in those schools, then when it was time to move on to secondary school we happened to end up in the same class - now this was not the reason we ended up befriending each other. The thing was, that having three different primary schools merge into one secondary school, they also had three former major bullying victims - so the other kids made a choice on which of us they were going to continue to pick on (I've only learned the full details on this recently). I was it - and the other kids gave the two other girls a warning to stay away from me if they wanted to be left alone. One if the girls decided to follow (she was the one who actually ended up telling me this, when I ran into her a little over a year ago in my old home town), the other girl (my now best friend) decided to do the complete opposite. She put herself in the crossfire and befriended me - and she's been my best friend ever since.
Our friendship is very sibling-like. We love each other dearly, have a fierce protective side when it comes to each other, but we also have the ability to really drive one another up the wall - and we both are very aware about that fact, often tend to point it out to each other. Interest-wise we are at about 50-50, there are plenty of things we share, but also plenty of things we don't. I know that I trust her with my life.
When it comes to friendships for me, most of them tend to either be rooted in history (as in people I befriended while in school etc. that I stayed in touch with regularly even after moving) or interests (mostly nerdy interests). I tend to only really consider people a friend if I feel I can trust them with pretty much anything (coming from a background of bullying it's important for me to suss out early on how trustworthy the people I meet are - whether they accept and like me for who I am) - if not, then they are an aquaintance that I sometime spend time with (I don't mind spending the occasional time with people who are fun, but if I don't feel I can really trust them, then I will never consider them as an actual friend).