That executes the code in a local scope, so that any variables defined within that block don't leak outside of its namespace.

Define an anonymous function that takes no arguments, and execute it without assigning it to a variable. It's often used in javascript to keep things out of the global scope.

This works even cooler when you want to minify your code:

MyLongClassName = function() {};

function(_c) { _c.prototype.method1 = ...; _c.prototype.method2 = ...; }(MyLongClassName);

This allows something like YUI compressor to shorten _c local variable for every method definition, rather than store "MyLongClassName" for each one.