A friend, to me, is someone who is willing to stick by you when you are at your worst or lowest. It's easy to have good times together, but being there when someone is feeling like absolute **** or acting up because of other circumstances, and remaining loyal, is something I find is very, very rare, but once I find it, it's something I will guard fiercely.
I also consider a friend someone who is willing to be truthful, even if it hurts me. I don't wanna be lied to or pacified in any manner.
We do not have to have a lot in common. In fact, I like a bit of diversity between me and others because I can learn from them about things that perhaps I've not yet been exposed to. If I can have the above as a solid foundation, then chances are I'm going to love the heck out of the other person, though I understand fully finding the above is no easy feat to start with.
An acquaintance is someone who I can be friendly with and we're generally on good terms with the other person, but it doesn't really go beyond that.
As for online vs. RL: I have no distinction here. The same applies to my interactions in both areas. This isn't to say that I think every online person I come across is gonna be a shining beacon of truth lol; I get people put up facades online, and you know what, I don't care lol because more often than not a lot of us, myself included, treat the internet as an escapism. But I have found friendship so strong over this form of communication that I would eviscerate anyone who dare tried to tamper with it, for example, I have a best friend over in Canada that I visit once - twice a year and we met online before RL. I consider him to be just as valid and just as precious as I do my RL friends because he's been there for me when I've been absolutely ****ed over, and stood resolute through it all. Generally, my rule is if you dare to see me as a real person beyond the monitor before you, then I will treat you as such, but if I am just an ~internet girl~ who is an avatar and nothing else, then I'll see you the same way--no hard feelings at all.