I've read those terms in a couple of lens reviews, and not even Wikipedia has an article about it.
So my questions are:
- What is back/frontfocus?
- What causes back/frontfocus?
- Is there any preventive measure to avoid getting back/frontfocus?
Front focus is when the lens and camera focus in front of your intended focus point. Your subject will look slightly out of focus and something in front of them will be razor sharp in focus.
Back focus is correspondingly when something behind them is in focus, instead of your intended subject.
As to why.. it could be mis-aligned, mis-calibrated equipment. It could be you or it could be your subject. You or your subject could have moved slightly after focusing and this would also cause front or back focus.
In terms of why it would be this way for the equipment:
There's a host of why, but most of them boil down to something being not quite calibrated, aligned, or working 'properly'.
Modern choices to fix it usually include one of three options that I'm aware of:
Which of these is an option, typically depends on the 'level' of camera you have. More entry levels (like my Nikon D3100) have no adjustment options. The Pentax K-x (mid level), for example, offers a global adjustment and the Pentax K-5 (mid-upper level) offer a per lens adjustment.