Facebook is "similar" to myspace, but it is definilty not the same. Assuming you are familar with myspace and the things you can do with it, I will tell you that facebook does not have the types of features myspace has. You can not pick your own backgrounds, add music etc.
Facebook is a networking system though. You can connect and "add" friends to your account. You can post pictures, leave comments, and send messages to other people. Facebook focuses on your profile information-such as interests, about me, etc. Everyone's facebook account looks basically the same. It also gives you the chance, like myspace, to join groups and set up events. One thing which is different is that facebook has a "home" page which comes up when you log in, advising you what changes have been made to people's pages, who added pictures, who commented on whose page, etc.
Facebook used to be for college communities only, but is now opento the public. Some people prefer it over myspace, but I prefer myspace.
Hope this helps!