In Mac OS X, you create a .zip file by Control-clicking or right-clicking any Finder icon and choosing "Make Archive of [the icon's name ]" from the shortcut menu. You decompress a .zip file by simply double-clicking its icon in the Finder.
Sometimes, however, you'll encounter compressed files on the Mac that end in .sit . You decompress such files with StuffIt Expander, a free download from www.stuffit.com.