Is your school blocking some websites? Would you like to access Facebook while you're in school?
Unblockers are 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 in your place, downloads the page, and forwards it to you. So the site you want to access only sees the unblocker's IP. So actually you are surfing sites indirectly. School firewalls think you are just visiting the unblocker, not the real site.
Another way of unblocking Facebook at school is by setting up your own unblocker tool. This is harder than just using an unblocker but it pays off because if they block it, you can make another. Explaining how to do this is not in this guide, do a search for "How to make your own proxy" you should find some guides. You can also earn money from this by letting people advertise on it.
Other ways to access Facebook are to use sites that translate pages, such as Google Translate or Babelfish. You can also try Google's cached copy of the site. However, you can't to log in that way, so it's still better to use an unblocker for Facebook, however these tricks work well for many other sites.