Depth of field is one of the most crucial things to understand if you ever want to become more than a snapshoot photographer. Bob Atkins has a decent article which attempts to explain what it is, with some info about how it works with digital SLRs like the EOS 10D.

“Depth of field is also NOT directly related to background blur. Depth of field equations tell you over what range of distances objects will appear to be acceptably sharp (or at least not unacceptably unsharp). It tells you nothing about how much blur there will be of objects well outside the depth of field. That’s governed by different physical parameters and determined using totally different equations, and it’s something I intend to address in a future article.”

Website: Bob Atkins- Digital Depth of Field