Declaring JS Functions in Two Ways : What's the Benefit?

I've come across JS code in which I see functions being declared in two different styles:

Method 1

function doSomething(){

   alert('This is one way of declaring a function');

}

Method 2

doSomething = function() {

    alert('And this is another way of declaring the same function');    

}

So what is the difference between the two and is there a case scenario where it would make sense to follow one method over the other?

PS: Sorry if the title seems vague. I didn't know the technical term used to describe the way the above two functions are declared.