Friends and Aquaintances

so you have friends and then you have aquaintances..when exactly does one cross the line and become a friend versus an aquaintance? What exactly is the deciding factor? I mean, lets discuss this. What makes someone a friend and NOT just an aquaintance?? Well, for starters, I talk to my friends on the phone..sometimes, we email back and forth and maybe I talk to them online too..I also tend to visit them or have them over my house on a regular basis..most of them are my age, we have things in common...aquaintances on the other hand, might run in the same groups as I do. I might see them if I go someplace that they would also go to..but I probably don't do all those things that make them an outright friend..so I'm confused..why can't some of my aquaintances seem to figure this out?? Why do they get upset when I don't call them when I'm around?? Not like I even have all their numbers or anything!! It just bothers me!!!