What does this "(function(){});", a function inside brackets, mean in javascript?

Possible Duplicates:
What does this mean? (function (x,y)){…}){a,b); in JavaScript
What do parentheses surrounding a JavaScript object/function/class declaration mean?

Hi All

I don't know what the following does:

(function(){
  // Do something here
  ...
})(someWord) //Why is this here?;

My questions are:

1. What's the meaning of putting a function inside brackets .i.e. (function(){});?
2. What do the set of brackets do at the end of a function?

I usually see these in jquery codes, and some other javascript libraries.