From what I understand, hard drives are a platter of sectors that could be written with data by magnetically setting individual bits on the disk to 1 or 0, organizing those bits into bytes of more meaningful info, and going up from there to KB, MB, and GB. The spindle on a hard drive interacts with the bits, writing or reading their state of 1 or 0. There is an index that the drive refers to to find sectors associated with a specific file. When you delete something, the HD notes that all the sectors containing that file are "deleted," and pretty much ignoresthat file...until the drive needs the room to write a new file. At this point, it will write over the sectors of the "deleted" file with the new file.
If you actually want to truly delete a file, you have to use a program that writes a random array of 1 and 0s over the sectors. Even then, advanced hardware and software can tell what was written there before. A military grade wipe is something like 30 random 1 0 writes over the disk.
data is recoverable from scratched, bent, shot, and even magnatized disks. Thermite does a good job though.
Feel free to make a fool of me. This was picked up by osmosis so a lot of it is assumed.