Does your school not want to allow you to access a specific site while you're in class? Would you like to know how to access Facebook at school?
Unblockers are like a gateway that pass information between your computer and the site you want to go to. The unblocker visits the site in your place, loads the page, and forwards it to you. So the site you're trying to visit never sees your IP. So actually you are browsing websites indirectly. School firewalls think you are just accessing the unblocker, not the real site.
Another way of unblocking Facebook at school is to make your own unblocker site. This is more complicated than just using an unblocker but better in the long run because you're in control. Explaining how to do this is beyond the scope of this article, but if you search for "How to make your own proxy" you should find instructions. You can also make some money from this if you put ads on it.
Other ways to access Facebook include by using translator sites, for example Google Translate or Babelfish. You can also try using Google's cache copy of the site. Unfortunately, you can't to log in using these methods, so you're better off using an unblocker for Facebook, though these tips work well for most websites.