File compression is used to reduce the file size of one or more files. When a file or a group of files is compressed, the resulting "archive" often takes up 50% to 90% less disk space than the original file(s). Common types of file compression include Zip, Gzip, RAR, StuffIt, and 7z compression. Each one of these compression methods uses a unique algorithm to compress the data.
So how does a file compression utility actually compress data? While each compression algorithm is different, they all work in a similar fashion. The goal is to remove redundant data in each file by replacing common patterns with smaller variables. For example, words in a plain text document might get replaced with numbers or another type of short identifier. These identifiers then reference the original words that are saved in a key within the compressed file. For instance, the word "computer" may be replaced with the number 5, which takes up much less space than the word "computer." The more times the word "computer" is found in the text document, the more effective the compression will be.
While file compression works well with text files, binary files can also be compressed. By locating repeated binary patterns, a compression algorithm can significantly reduce the size of binary files, such as applications and disk images. However, once a file is compressed, it must be decompressed in order to be used. Therefore, if you download or receive a compressed file, you will need to use a file decompression program, such as WinZip or StuffIt Expander, to decompress the file before you can view the original contents.