After reading the quote below on the Casey Anthony trial (CNN) ,I am curious about where deleted files actually go on a hard drive, how they can be seen after being deleted, and to what extent the data can be recovered (fully, partially, etc).

"Earlier in the trial, experts testified that someone conducted the keyword searches on a desktop computer in the home Casey Anthony shared with her parents.

The searches were found in a portion of the computer's hard drive that indicated they had been deleted, Detective Sandra Osborne of the Orange County Sheriff's Office testified Wednesday in Anthony's capital murder trial."

I know some of the questions here on SO address third party software that can used for this kind of thing, but I'm more interested in how this data can be seen after deletion, where it resides on the hard drive, etc. I find the whole topic intriguing, so any additional insight is welcome.