Prosumer Cameras: A Serious Hobby
At this point, we need to mention the difference between full-frame and cropped-sensor cameras. In a full-frame DSLR, the digital sensor is the same size as 35mm film; you can use your old film lenses and get the same magnification. Most digital cameras, however, have a smaller sensor, so they crop the picture; Nikon calls these DX cameras. When you buy a lens, the length marked on it is actually the focal length for a full-frame camera; using one of Nikon's cropped sensor cameras gives you a crop factor of about 1.5, meaning you'll get a picture similar to if you were using a 50% longer lens. Notice that I say similar, not the same as, because the depth of field doesn't actually change.
Full-frame lenses tend to be big, heavy, and expensive, because the light from them needs to cover a much greater surface area. DX cameras also tend to be faster; Nikon's prosumer line can take considerably more photos per second than their professional line. These bodies are as good as you can get with a DX camera, and when the D300 came out, many people considered it to be Nikon's best camera for professionals as well as semi-pros. The D700 straddles the line between prosumer and pro; this is essentially the same camera as the D300 (it even uses the same size body), but with a full-frame sensor. Prosumer cameras tend to be the point where you begin using CF memory cards rather than SD. At this stage, camera bodies are generally sold by themselves, under the assumption that buyers already have lenses to use with them, or at least have a specific lens in mind.