The first type are the "normal" friends - they're good friends until you reach a turning point in life and you end up struggling. I don't blame them, they're not strong enough to withstand all the things society says about you so they leave. These usually go with the crowd.
The second are the "best friends" - you can trust them with mostly anything, your secrets, dreams, hopes. Most of the time, you'll have to be careful of how much you're going to trust them because they're so seemingly trustworthy.
The last are the "true" friends - they don't ask much of you, they're always there for you, and you can always rely on them when you're at the end of the road. Of course, they might not be as entertaining as your "best friends", but you can always count on them to have your back.
I'd say the most important trait is how trustworthy and loyal they are.
Sure, they might not be the one you choose to hang out all the time with, but they'll always be by your side, supporting you.
