Nikon lens difference?

I went to a camera shop today & was looking at the
Nikon AF NIKKOR 50mm f/1.8D Lens and
Nikon NIKKOR AF-S 50mm f/1.8 Lens

The guy told me there the exact same lens but ones just more expensive ($299 vs $129)

What's actually the difference between them?

On your Nikon D5100, you'd need the AF-S model for autofocus. For future reference, here's a webpage that lists all compatible lenses for your current camera.
http://en.wikipedia.org/...ocus_motor

The AF-S version has the built-in focus motor, required to permit autofocus on non-motorised bodies (D3xxx/D5xxx).

The AF version fits all Nikon bodies, but won't allow autofocus on budget models with no body motors.

The af-s version is ( optically ) better and with built-in autofocus motor