Who can be a true friend to you?

What traits do all your true friends have in common? What do you have to do with a person in order to consider them a true friend? What are true friends willing to do for each other? What are the criteria you use in order to move a friend into your own personal true friend category? Do you have any litmus tests? If so, what are they? Can someone you have never met in person be a true friend?

For extra credit (that’s a joke) describe your hierarchy of friends from people you happen to know peripherally on up to true friendship.