Where do deleted items go on the hard drive?

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 Super User 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.

In general, deleted files don't go anywhere. They remain on the disk exactly as they were until they happen to get overwritten. When they are deleted, a link to it is simply removed from the file system structure.

Imagine a library in 1970. You had all the shelves with the books on them and you had drawers with cards that could tell you where the book you were looking for was located.

On a hard drive, you have a table (the drawers) that's separate from the files (the books).

Your operating system references this table when it needs to find data. It then goes to the location of the book with it's read head or whatever the device uses and reads the data there.

When a user decides to delete a file the computer just erases the contents of the table for that location, this basically puts a blank card for that file in the table. because it would take more time and power for the computer to go to each location and actually erase the file, it just leaves it there.

Then, because the book is still physically in the library, even though it's not on their records, it's possible for special software to read all of the books and find out what's recently been deleted (As long as it hasn't been written over since then!)