There's a page at [focustestchart.com][1] that goes into enormous detail about this. 

The quick summary: back focus is when the focus plane is further back than you were expecting it to be (so your subject is out of focus, but something behind it is sharp). This is usually operator error, but might possibly be a problem with the lens; the page linked above gives you a way to distinguish between the two.


  [1]: http://focustestchart.com/chart.html