What type of programs reduce the amount of disk space a file requires?

Answer 1

These are called compression programs. They take a file (or a series of files) and compress the data. Be aware that compressing files does not always reduce the file size drastically, especially if the file has already been compressed or is in a compressed form such as some video formats. Trying to compress them can even result in a file being bigger than the original.

Many of these programs accomplish compression by finding repeating fragments of characters or data, replacing it with a unique code, and include the code and data fragment into a dictionary in the resulting compressed file. Decompressing the file is done by extracting the dictionary, scanning the file for dictionary codes and replacing them with the data fragment. The most dramatic effect is when compressing many files into one archive. Compressing a single file often will have little effect on the amount of data it takes up on the hard drive.

There are many programs that perform this function: WinZip, WinRAR, and 7Zip, zip, gzip are common and widely used. Some programs have options tospecify how thoroughly to scan the original file and how much compression should be attempted. The more compression desired, the longer it takes to scan and study a file.

Some of the compression programs also allow specifying a password to be use for encrypting the file and the password is required to extract information afterwards.

Microsoft Windows versions as well as some other companies offer the ability to compress a whole drive or directory.