Kind of difficult to answer this in a single post - there's so much you can do with layer styles - but this will give you some idea:
Photoshop Layer Styles
As to whether they're worth downloading: well, any style that you can download you could (theoretically) make yourself, but sometimes it's just a lot quicker and easier to grab a pre-made style than to spend hours tweaking settings yourself - after all, it may be very satisfying to make your own pizza from scratch but if you're really hungry you just want to call Domino's.
[PS: Tamlin is not affiliated with Domino's Pizza. Other pizza outlets are available...]