7-zip handles compression and decompression of files. 7-zip is one of the first utilities to install on your computer.
7-zip is the best compression utility for Windows. There are versions of 7-zip for Unix and Linux. 7-zip is one of the first utilities to install on your Windows machine. Some versions of Linux automatically install an equivalent.
When you install all the extras you need on Windows, Linux, Unix, or Mac (any of the many incompatible Mac operating systems), you end up with files compressed as zip, .gzip, rar, 7z, bzip2, and tar. Somehow you have to unpack them. You want an unzipper but more than an unzipper, an unpacker that will handle all the useful formats.
Some operating systems arrive with some unpacking built in. Windows used to have nothing then, by the time zip files went out of fashion, Windows unpacked zip. Rar, bzip2, and 7z compression all produce smaller files than zipping. At most download sites, 7z and bzip2 are offered as alternatives to zip to save network time and disk space.
Stuffit Expander used to be the most flexible way to decompress files but you have to pay a licence fee to compress stuff.
When you create your own archives for backup or distribution, you want the higher compression ratio of the modern compression formats but you do not want to limit your options for decompressing the files when you want to retrieve the content. 7z is free and open so you can use it anywhere anytime. 7-zip is free and open so you can install it on any machine. You can ship 7-zip on your backup CDs and DVDs so you have the software to decompress the backups when you need them.
7-zip for Windows is just 0.8 megabytes. You can have all the versions of 7-zip on your USB stick in a few megabytes. You can then save hundreds of megabytes in your USB stick by having all your other files compressed into the 7z format.
Keep the Windows and Linux versions of 7-zip on your USB stick so you have something to unzip, un7z, unbzip2, and unrar all those other files.