Good question @Shippy . I have thought of posting variations on this question many times on Fluther.
I think I operate in terms of degrees of friendship. There are friends/buddies (mostly from work, but also locals in town) with whom I go drinking or out to restaurants, dancing, etc. From them I expect companionship if we happen to be hanging out in the same place on the same night. We buy each other drinks and talk and laugh. Otherwise, I don’t really talk to them if we’re not hanging out, and our conversations don’t get too personal, but that’s mostly just a function of not spending much time together.
I have a very few old friends whom I have known since high school (my best friend in high school and my girlfriend from high school, actually), and while we don’t hang out in person because we live very far apart, I expect them to know the real me. We have deep conversations and even if they are weeks or months apart, it’s like time never passed because we truly know and accept each other and we have shared so many formative experiences that still inform our frames of reference.
I have some professor friends/mentors (a very grey area). We might try to get together if we are in the same city at the same time, but mostly we communicate infrequently by email. I wouldn’t say I expect anything from them, but I have asked them for career advice, balancing work/personal life advice, perspectives, and occasionally recommendations for grants/scholarships, etc.
Finally, I have two best friends whom I consider as close as siblings. I feel comfortable talking about pretty much anything with them—things I don’t talk about with anyone else IRL or online, including my therapist. I didn’t expect this from them, but both gladly lent me money over the summer when I was broke as a joke in Paris and had to choose between eating lunch and eating dinner most days. I view this kind of friendship as ‘true’ friendship, the kind that involves a mutual obligation to help. I would help them out in any way I could, emotionally, financially, logistically or otherwise, because I know they would do the same for me.