Best way to define a function?

I'm always learned to define a function in JavaScript like this:

function myFunction(arg1, arg2) { ... }

However, I was just reading Google's guide to Javascript, it mentioned I should define methods like this:

Foo.prototype.bar = function() { ... };

Question: Is "Foo" in the example an Object, or is it a namespace? Why isn't the Google example the following code (which doesn't work):

prototype.bar = function() { ... };

UPDATE: In case it helps to know, all of my JavaScript will be called by the users browser for my web-application.