Is your school censoring some sites? Is Facebook blocked, but you'd like to visit it? You'd like to visit Facebook, and its being blocked by the school administrators?
Unblockers are like a gateway that pass information between the computer you're on and the site you're visiting. The unblocker accesses the site instead of you, downloads the page, and sends it on to you. So the site you want to be on only sees the unblocker's IP. So actually you are visiting sites indirectly. School firewalls think you are just accessing the unblocker, not the site you actually want.
Another way to unblock Facebook in school is to make your own unblocker site. This is more complicated than using an unblocker but better in the long run because you can make more of them. How to do this is too much effort to explain here, if you do a search for "How to make your own proxy" you should find help. You can even earn money from letting people use it if you put ads on it.
Other ways to get on Facebook are to use translator sites, for example Google Translate or Babelfish. You can also try Google's cached version of the site. Unfortunately, it won't let you to log in that way, so it's still better to use an unblocker for Facebook, however these tricks are good for many other sites.