Generally, if a lens is marked as autofocus, that means that when you put it on a camera that is capable of autofocusing, the camera will focus it.
Naturally, if you have an old camera body that cannot autofocus, it doesn't matter if the lens is capable. Similarly, if the lens is manual focus, an autofocus camera body won't be able to focus it.
Since you have a K100D, you have a body that will perform autofocus (obviously
), so any autofocus lens will focus with it, while any manual-focus lens will need to be focused manually.
Some lenses actually have motors inside them that do the focusing themself, but this is generally not worth noting yet in the Pentax world, as I don't think any Pentax lenses are actually out yet with this setup. The upcoming $900 16-50mm F2.8 and the $1,000 50-135mm F2.8 are, I think, the first lenses that will have this. I'm not sure at the moment how these will work on the K100D as their K10D is the first Pentax designed for this setup - it might be something as simple as a firmware update to let the K100D work with these, it might not be possible due to missing connections... I'm not sure.
