Pentax has a long history of K-mount lenses. This is useful since even their current cameras work with all legacy k-mount lenses ever produced!
of course, figuring out which lens is which gets kind of confusing sometimes as there are so many designations. Here’s a great reference breaking down the various generations of K-mount lenses and their nomenclature:
K – “King”, Manual focus, 1st generation of K-mount lenses.
M – Mini, Manual focus, 2nd generation of K-mount lenses, smaller than the K models.
A – Auto aperture, manual focus, 3d generation of K-mount lenses, comes with electrical contacts för the aperture.
F – auto Focus, 1st generation of autofocus lenses
FA – auto Focus Advanced, 2nd generation of autofocus lenses, has a more advanced built-in lens chip.
FA J – auto Focus Advanced Junior, as FA lenses but without aperture rings, for consumers with cheap build quality (plastic lens mounts).
D FA – Digital auto Focus Advanced, as FA lenses but with extra coating and more telecentric design.
DA – Digital Autofocus, for the APS-C sized sensor
Star(*) = Professional lens, pro-quality, exclusive optics.
Limited = Exclusive quality prime lens, full metal build quality, compact design.
I actually can’t take credit for assembling the list, it comes from Roland on DPreview:
http://forums.dpreview.com/forums/readflat.asp?forum=1036&thread=23026442