You can use older, manual focus lenses on dslr.
Why would you? It's cheaper.
Why wouldn't you: you have to fiddle with focus and other stuff a bit more.
Masses of detail at PhotoNotes.org
Photojojo's "How To."
Wired has a great article that flags how manual focusing can be trickier.
Arthur H. Bleich at DP Corner swears by Steven Gandy's more expensive adapters.