Friendship is the feeling between two or more people who care about each other. There are many types of friends, like mutual friends, people you know through other friends, casual people that you hang out with now and again, and best friends: the people you would do anything for.
Some friendships are not physical ones, for instance, an ibf (internet best friend). These are friends that you met online that you have an instant connection with. You call them and talk about all your problems, but they are not there in person. Sometimes these are the best kinds of friends.
Most times best friends think each other are jerks to begin with. They think poorly of each other, but that soon changes. They start hanging out with each other more and become best friends. They laugh with you and make fun of you in a loving way.
Overall, friends are the best and you should keep them around.