Is your school censoring specific sites? Is Facebook blocked, but you have to unblock it? You wanna visit Facebook, but its being blocked by the school?
An unblocker is like a gateway that pass information between the computer you're on and the site you're trying to visit. The unblocker visits the site instead of you, loads the page, and sends it on to you. So the site you want to be on only sees the unblocker's IP. So, basically you are surfing sites in an indirect manner. School firewalls only see you visiting the unblocker, not the site you actually want.
An alternative way to unblock Facebook at school is to set up your own unblocker tool. This is more complicated than using someone else's unblocker but better in the long run because you can make more of them. How to do this is not in this guide, but if you search for "How to make your own proxy" you will find info. You can even make money from your unblocker by letting people advertise on it.
Other ways to get on Facebook include by using sites that translate pages, such as Google Translate or Babelfish. You can also try Google's cached copy of the site. However, you probably won't be able to to login using these methods, so you're better off using unblockers for Facebook, however these tricks are good with most other sites.