This is simply a substitution cipher, the most common method of securing computer data. It's not the same thing as a code in which a randomly chosen word or phrase is used to represent some other word or group of words. The mathematical formula that is used to turn plain text into cipher text and back again is called an algorithm. Within the category of data encryption are subcategories such as e-mail encryption or encryption of the data on your hard drive. You specify which files to encrypt or decrypt, and the program processes them in sequence in one run.
Some products offer both batch and stream processing.
With apologies to the Coasters, take out the papers and the trash, yakety yak. At the CIA, papers are shredded and then burned, providing a fairly good level of security. Your agency probably routinely shreds important documents, too. It's wonderful if your office has a program of donating obsolete PCs to some worthwhile cause, but remember that you may have confidential data on that PC or on old floppy diskettes. It's a bad idea to give anyone access to encrypted files.