If you see the twitter\'s bootstrap application.js file that exists in all documents, you will notice something the following code:
!function( $ ) {
...
It's a self executing function:
!function(x){}(y)
x is the parameter of the function and y is the parameter you pass in the auto-call of that function.
! is just a visual guide that tells you at a glance that the function is self executing. Another common practice is to wrap it in parenthesis instead. (function(){}())