A RAR file is a Roshal Archive Compressed file. It was developed by Russian IT engineer Eugene Roshal and is primarily used to create an archive of multiple files, compressing them into a smaller package size. A RAR file requires a separate software program to decompress it.
File compression is a way to reduce the number of bytes in a file. This allows the compressed file to be transmitted faster over a network and reduces the amount of space it takes up on a disk. Compression is done by eliminating some of the redundancy in files by looking for patterns and replacing them with something that uses less bytes.
For example, an article that uses the phrase "carbon dioxide" several times could replace this phrase with the letters "cd." When the file is compressed, every instance of "carbon dioxide" will use 12 less characters than the original file, which can save significant space when applied to all instances of redundancy in a file.