They mean the exact same thing. It's just syntactic sugar. The latter is IMO more revealing of what JavaScript is really doing; i.e. "sum" is just a variable, initialised with a function object, which can then be replaced by something else:
$ js
js> function sum(x,y) { return x+y; }
js> sum(1,2);
3
js> sum=3
3
js> sum(1,2);
typein:4: TypeError: sum is not a function
js> sum
3
