I just learned something new that I want to share with you. My husband bought me a new Nikon D-5000 camera body and a Tamron 18-270 lens. I have been using my Nikon D50 using my three lenses: 70-300, 28-200 and 18-55. He was hoping to buy the body only and use my existing lenses. Turns out that the older cameras drive the lens through the motor in the body. The newer bodies don't have the motor to drive the lens. You have to buy a new lens because the lenses now have their own motor but get the power from the body of the camera. This gives you quicker and better focus...or something like that. So if you are planning on getting a new camera body thinking your old lenses will work...take the lenses with you and have the sales person check them out. Also, if you have an external flash the Nikon Speedllight SB-600 will work. That is the one I used with my D50
It depends on the lens & camera - the camera body may or may not have the autofocus motor to drive the lens. I also learned that the hard way - I started out with a Nikon D60 when I first started as an amateur professional a few years ago (I got the D60 right when it came out.) I bought a Nikon 50mm fixed 2.8f lens, put it on the D60 body....and seriously thought the lens was broken because it wouldn't auto focus, or something was wrong w/my camera body :) A photographer friend had to explain to me that I had to gasp>> manually focus using this lens because the lens doesn't have an autofocus motor in it, and neither did my camera body!
I learned my lesson and just last month moved up to the Nikon D300s which has the auto focus motor in the body, so no matter what kind of lens I use, I'm good to go, everything will auto focus with that body.
On a side note - that 50mm lens ended up being my FAVORITE lens, it was only $100 and I use it all the time. I think it really made me learn more about focus and depth of field by having to manually focus. Everyone should take that camera off auto focus once and while and work your skills on manual focus!