I'm not sure there is one definite definition of a friend, or a best friend, or even a BFF, but you knwo them when you really think about it. You just know who they are. When something is wrong, who do you call first? Who answers? Is there someone who understands you better then everyone else? Or remembers all the little things that make you you? These are the people you will want in your life later, don't forget them.
I've got friends that I see regularily. We go to the mall or watch movies, or even just cook supper and watch a TV show. We have things that we bond over and certain habits that we share. These are great people to have around and necessary. These are the people who will make your daily life fun and keep you busy on the weekends. They are the ones who know where you get your hair done and your favorite place to eat, but they may not know you're childhood details, your fears and memories, even your other friends.
Your BFF knows the big details, but likely not the little ones. I remember the big fight you had with your parents when you moved out, they know what your first car was and remember your first love. They annoy you and drive you crazy, but they are alwasy there anyway. They care no matter what happens or how long it's been since you've seen each other. They've shared so many experiences with you that you are forever bonded in some way. They may not return every message, but they respond to the important ones. They might now know what your favorite supper is now, bu they know what you are allergic too. They remember your goth phase and when you wore belly shirts or had blue hair, they know you're parents on sight and remember where they live. These are the people who will know how you feel after a break up, who will understand your need to drink a coffee from a certain place to ease the pain. Talking to them after 2 years is teh same as talking to them after 2 days. Some things never change, and while people do, you're connection with them can remain strong and uninterrupted. These are the friendships that remain throguh the changes, and that grow and change with them rather then falling apart. The friends you coffee with today may not be the friends you coffee with next year, but the ones who truly know you will be around in 10 years and still remember that blue hair...
This is just a reminder that sometimes we over look those who are most important, although their staying power after being overlooked is exactly what makes them so important in the first place. Don't forget those who you may have grown distant from because life changes makes it harder, if you are true friends you can find time, and you can share those differences as once you shared likenesses. It may help you grow, it helps you remain grounded and remember who you are. When you sit on your porch in 50 years thinking over your past and remembering your friends, they are the ones you will miss the most!