A computer's hard drive stores data, and maintains an index of records. When you save a file, particularly a large one, it is scattered roughly the hard drive in bits and pieces. Files are automatically created by browsers and operating systems. Once you open a file, the hard drive checks the index, then gathers the bits and pieces with reconstructs them.

When you delete a file, the links between the index and the file disappear, signaling to your system that the file isn't required any longer and that hard drive space able to be overwritten. But the bits and pieces of the deleted file wait on your computer until they're overwritten, and they can be retrieved with a data revival program. To remove data from your hard drive permanently, it wants to be wiped clean.