What type of lens do I need so I can auto focus on a Nikon?

I was looking to purchasing a D3100.It's cheap and is very good from what I hear. It however doesn't have a built in focus motor like the D80. I don't know much about dslr's at this point. Do I need to buy a lens that has AF listed for it to auto focus, or maybe AF-S? From what I have read, there are lenses with a motor system of some sort for those cameras that do not have a built in motor.

To autofocus you need a lens that has its motor built into it. For Nikon brand lenses, those are the AF-S lenses. Different brands have different alphabet soup for this feature. For Sigma it is HSM (hyper sonic motor).

Any AF-S or AF-I lens will auto-focus on the entry level Nikon dSLR's (under $1, 000)

If you step up a little and buy the Nikon D7000, you can use any of the Nikkor AF, AF-I or AF-S lenses in the auto-focus mode

Ah, keerok nailed.

Use the Ken Rockwell chart and you will never have to worry about which lens to buy or not

You might like this one

Nikon 55-300mm f/4.5-5.6G ED VR AF-S DX Nikkor Zoom Lens for Nikon Digital SLR

5.5x Telephoto Zoom lens
Nikon VR II Image Stabilization; Tripod Detection Mode
HRI (High Refractive Index) Lens Element
2 Extra-low Dispersion (ED) Elements
Exclusive Nikon Silent Wave Motor (SWM)