Nikon is one of the "big two" in the world of SLR manufacturing.  They've continued their prominence from the film-based SLRs, in part, by making the transition from film to digital as painless as possible.  Many recent lenses built originally for film can be used on Nikon digital cameras, thanks to their decision to use the same lens mounting format.  The body styles have remained relatively the same as film cameras, with the addition of controls and a monitor on the camera back.

 

The current series of Nikon digital SLRs includes (from highest price to lowest): D3, D2, D300, D200, D80, D40.  In general you get what you pay for, with higher image resolutions, more flexibility and greater speed in the more expensive models.