The high school friend
There's a good chance you've had this friend much longer than high school - this is the person that remembers when "the popular boy" at school made you cry, they picked you up off the bottom of your locker the first time your heart got broken and they were there to jump and squeal (if you're a girl) when your parents finally caved and let you get a car. If you're me, which clearly I am, this person also still knows me better than anyone could. Even though she lives a bazillion miles away and I talk to her like three times a year, I know that if I ever, EVER, need anything, we will just pick right up where we left off. It doesn't matter that we basically only have a Facebook status holding together our day-to-day knowledge, she gets me. I also am blessed to have a second great friend from high school that I get super-excited to see every time I visit the 'rents. I know that most people aren't lucky enough to still have this though, so it's likely that your high school friends are people you kept in touch with in college - mostly via Christmas/Spring break parties when you were avoiding your parents - and then lost touch with outside of Facebook when you hit true adulthood.
The college friend(s)
Ohhhh the college friends. Now these people know you, but they know the absolute crazy side of you. They have blurred memories of your too many to count drunk nights together, they know the secrets you pray they don't remember and they may or may not have been there when you made that life path decision that really changed everything. Your college friends are likely what I consider my high school friends to be - your hands down best friends. So many people I know today are still in touch with their best college friends and hold those people to be the most sacred - though unless you've stayed in the same few states vicinity, it's really getting hard to keep up with them. I do have a few really crucial college friends that I am lucky enough to say are still a part of my life. Some of them more than others, but the ones that really matter are still here!
The work friends
This is always a really interesting cluster of friends. Sometimes, your friendship never leaves your cube/cafeteria at the office. Sometimes, you are lucky like I have been, and moved those friendships past 8-5 and have even pulled your significant others into the mix. These people know a lot of the inner workings of your life, because hell, you spend more time awake with them than anyone else in your life! This can be a scary cluster of friends though because you really have to be careful who you trust and who you let in, but once you're sure, nothing is better than a conspiratory smile as you pass by each other in the kitchen because you know what happened last weekend.
The friend of a friend
This has always been one of my favorite categories because some of the most fantastic friends I've made have come to me this way. This person was someone else's high school/college/work friend and you were in the right place at the right time and found yourself a new
I think the hardest thing we deal with though, as we progress through all of these people, is realizing when the time is to keep them, when the time is to let them go and that you don't always get to have a "best friend" in the high school/college sense of the words. There are people who came in to your life that you're probably glad are gone, those who you really wish had never left, and then those that are here and maybe you aren't quite sure what to do with them.
Recently, I've been really trying to look at all of these people and remember what they taught me, because I can promise you that I learned something from each and every one of them. They may have taught me about the person I wished I was, the person I'm glad I'm not or even just that taking the time to envy someone else's life is just not worth it - if their life looks that great then they are just really good at hiding the crap they're dealing with. Every now and then, I also have to believe I've taught them a thing or two along the way.
I hope reading this brought thoughts of people past and present to mind that you haven't thought of in a while. Enjoy the memories kids.