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front or back focus is a mechanical deviation as a result of misaligned between the image presentation and the sensor.
when the camera or you for that matter achieves focus, the image is focused on plane (a two dimensional space). that plane is overlayed directly on the sensor, and that is the image that is captured. the problem is, you are converting a three dimensional image into 2 dimensions, so sometimes things aren't perfect. because of manufacturing tolerances, sometimes the image is presented "correctly" (ie as intended focus) slightly in front of or behind the ideal focal plane (think of it as if you had focused manually and then leaned forward or backward ever so slightly). this results in a different portion of your image being in focus, rather than the intended area.
without diagrams and formulae, it's tough to get into the whole physics explanation. the bottom line is, ff or bf can be fixed used a mechanical method of physically changing the location of your sensor (micro adjustments). what amount of micro adjustments are needed can vary by lens or or even camera body (globally), which is why pentax has presented both abilities through the menu system.
how you determine what adjustments are needed is through the use of a focus calibration chart.
search the forums for several threads and articles on where to get one, how to make one and/or how to use one. you can also do a google search for similar information.
it's a fairly straightforward process, but it requires an attention to detail and some time. it takes me about an hour per lens, based on my penchant for checking, double checking and then triple checking, plus set up time.
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