Pentax has many lenses on it. Some of the Pentax bodies require the use of an "A" lens (usually marked as a Pentax-A), but most Pentax bodies which can accept a K-Mount lens can use every K-mount lens made since the mount was introduced back in 1975 with the exception of those with the DA designation which are designed for the smaller APS-C sensor of the Pentax DSLRs.
This means you have a very wide selection of Pentax (and non-Pentax) lenses you can use.
With adapters, you can also use most if not all of the M42 screw-mount lenses on Pentax cameras. Mike Butkus has lots of camera manuals over at his site, www.butkus.org; you can use these to see which kind of camera you want to get, or to get a manual for your new purchase. Leave him a donation if you end up downloading one of his manuals for your new camera. His manual collection includes more than just Pentax, so you can also get Canon/Nikon manuals there too.