Summary
A graphical file archive management tool, reads many formats. Can create zip files.
Features
Windows, like most operating systems, has had the ability to read zip files for years, so why would you go through the trouble to use a separate application? 7-Zip is nice because it handles more than just “Zip” files. For starters, it can open ISO disk images, “rar” files, as well as it's own highly optimized format. It can also open installer applications made with several popular application packagers, to extract the files used in the install.
One of the other nice feature with the shell integration feature, you can open any file with 7-Zip with the shell context menu option “7-Zip→open archive”. As long as it is a file format that 7-Zip can handle, it will open it, without you having to specifiy why type of archive it is. This is helpful because a lot of file types, such as java 'jar' or svg images 'sgvz' are actually just zip files with a different extension. The built-in archive processing in Windows will not do this.