Here is the quick rundown:
ANH is what you see most places: gray mouth, thin lined helmet details and the "normal" hand armor.
ESB were reused ANH sets with: black frown, thicker lined helmet details (and they used stickers), and the snowtrooper hand guards.
ROTJ used recast ESB armor (hence the softness of the edges) with the bottom chest trim added, different button configuration, and a resculpted helmet many have nicknamed droopy or the bulldog helmet.
One tidbit, the ROTJ stormtroopers are technically ANH cannon now. All the CG stormtroopers that were added in the ANH special edition are ROTJ troopers.
Please feel free to add anything I've missed, or anything I've misreported.