These lenses are designed specifically for Pentax's line of medium format cameras. They can be adapted onto K-mount DSLRs via an adapter, but this is typically not convenient as 645 lenses are much larger physically than a corresponding K-Mount lenses.
Series overview: -"A" lenses are manual focus (1980's) -"FA" lenses are autofocus (1990's) -"DA" and "DFA" lenses are optimized for digital and most feature silent autofocus and weather sealing.
The lenses in this category are all zoom lenses. You'll notice that no lens offers more than 2x zoom; this is for two reasons: to keep the size relatively compact, and to maintain good image quality.