You've deleted things, probably even things you didn't mean to. Did you know that there are ways to recover your files that even someone who doesn't understand the importance of backing up could employ? Even those files which you intentionally deleted on that reformatted hard drive are recoverable, you poor soon-to-be-RCMP-investigated soul.
These shocking truths are because deleting files from the recycle bin or formatting hard drives doesn't delete the contents of the files. The operating system maintains references to files on hard drives. When a file is deleted, the reference maintained by the operating system in a file system is deleted and not the content of the file on the hard drive. This makes the deleted file hidden from-- or invisible to-- the operating system, but the content of the file can be retrieved by software if the content of the file hasn't been overwritten by a newly created file. The same applies for formatting a hard drive in that only the file system declaration maintained by the operating system is deleted.
Deleting a file reference without deleting its content can be a double-edged sword. It can be used to retrieve an important file which has been mistakenly deleted. It can also be used in a harmful way by malicious tech savvies to retrieve sensitive files which the user intended to delete permanently.
There are a plethora of file-restoring utilities which specialize in different operating systems, file systems, and media. Should you find yourself in need of one, your search engine of choice can help.
When it comes to securely deleting files, your OS is likely to already have you covered. On Unix-like systems, you can use the "shred" command to delete the content of a file. OS X's "securely empty trash" option is also removes deleted data. Windows users will need to download third-party software like Freeraser or SDelete.
So the golden advice is before disposing your hard drives, flash drives, digital cameras and other storage media, make sure you wipe the personal files correctly and not just delete them.