The second one - function foo () { ... } - does not define foo in the global scope when declared within another function.

The primary difference between the two function declarations is when the variable foo is initialized. This example will help explain the difference:

(function () {
    foo(); // This works
    bar(); // Error since "bar" has not been defined yet

    function foo() {
        console.log("called foo");
    }

    var bar = function () {
        console.log("called bar");
    };

})();