Does your school refuse to allow you to access a certain site while you're in class? Is Facebook blocked, and you desperately need to visit it? You have to visit Facebook, but its been blocked by the school?
An unblocker is like a gateway that sit between your computer and the site you're trying to visit. The unblocker goes to the site you want on your behalf, downloads the page, and sends it on to you. So the site you want to be on never sees your IP. So in effect you are visiting sites in an indirect way. School filters only see you accessing the unblocker, not the real site.
Another method to unblock Facebook in school is to make your own unblocker tool. This is more complicated than using someone else's unblocker but it pays off because if they block it, you can make another. 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 help. You can even make money from this by letting people advertise on it.
Other ways to get on Facebook include by using translator sites, like Google Translate or Babelfish. Also try Google's cached copy of the site. However, it's not possible to log in using these methods, so you're better off using an unblocker for Facebook, however these tricks are good for most other sites.