Friends?

Are there such things as good friends and bad friends?
Friends can't meet up to your expectations all the time. All you got to do is accept them for who they are.
It's pretty much impossible be a perfect best friend. So what defines a bad friend?
I heard a quote that says something like: Friends are not necessarily those people who are always there for you. But even after moments apart, nothing changes between the two of you.
I thought I agreed with that, but then is it really?
Because a friend complained to me once that as a friend you need to be there for them all the time.
Otherwise you'd be classified as a bad friend.
What if your life circumstances changes, and you can't be there as much anymore? You still care for them, but not sure if they still care for you after you sacrificed their time for another person. Would that make one a bad friend?