Front Focus and Back Focus

I was reading a thread a bit ago where a person said something like "My lens tends to front focus a bit. I've been working with it and think I have it dialed in now, though."

What does "working with it" mean in this case? If a lens suffers from front or back focus, isn't that a calibration problem? How can a user work with it and dial it in? Other than playing with focus points, which is kind of a band-aid, I'd think, what do you do?

Educate me.

Perhaps he/she was talking about "AF Micro Adjustment". This is a feature available on a few Canon bodies (1DIII/IV, 50D, 5DII and 7D AFAIK). This allows making adjustments to the way AF system works with all lenses or particular lenses (if set up this way). If the lens's AF is off by a small margin, this feature can correct it without a need to calibrate the lens professionally.

Could someone maybe post an example pic of a front focus problem and/or back focus problem....I read so many threads on focusing issues but have a hard time understanding how one would identify this problem...i doubt Canon´s QC is so bad.

It is easy to identify. You focus on a high contrast object in front of you, you pump the AF button, and the camera locks in on it, but when you take the shot and bring it up on the monitor, you notice that actual subject material in front of or in back of the object in question is actually in focus.

The difference is caused by the error tolerances in light's path traveling to the AF sensors via the mirror/pentaprism VS the light from from your lens directly to the sensor. The newest cameras allow you to dial in step offsets for the AF engine to use when it talks to your lens to move the focus ring. In the "old days", you would send both your lens and body to Canon, and they would adjust one or the other or both.