The first statement depends on the context in which it is declared.
If it is declared in the global context it will create an implied global variable called "foo" which will be a variable which points to the function. Thus the function call "foo()" can be made anywhere in your javascript program.
If the function is created in a closure it will create an implied local variable called "foo" which you can then use to invoke the function inside the closure with "foo()"
EDIT:
I should have also said that function statements (The first one) are parsed before function expressions (The other 2). This means that if you declare the function at the bottom of your script you will still be able to use it at the top. Function expressions only get evaluated as they are hit by the executing code.
END EDIT
Statements 2 & 3 are pretty much equivalent to each other. Again if used in the global context they will create global variables and if used within a closure will create local variables. However it is worth noting that statement 3 will ignore the function name, so esentially you could call the function anything. Therefore
var foo = function foo() { return 5; }
Is the same as
var foo = function fooYou() { return 5; }
