On the Macintosh: StuffIt
The standard compression and archiving format in Windows is Zip. Although Macs can create and open Zip archives (using, for example, Aladdin’s StuffIt program), the primary compression and archiving format on the Macintosh is the StuffIt format. You can identify a “stuffed” file by the letters .sit at the end of its name, just as the letters .zip identify a Zip file.
Because StuffIt is a proprietary format, most of the software available for working with StuffIt archives comes from Aladdin (or is based on software modules from Aladdin). To survive in the Macintosh world, you’ll probably need, sooner or later, some of the following programs (all of which can be found at http://www.aladdinsys.com):
Expansion. StuffIt Expander is a simple utility that can expand any StuffIt archive, along with many other formats, including Zip. (StuffIt Expander is included with every new Macintosh; use your Apple menu → Sherlock command to locate it.) When you download a file from the Internet, most email or web browser programs automatically launch StuffIt Expander to expand the newly arrived software.
If you receive a file attachment by email that doesn’t automatically expand in this way, you may have to drag the file’s .sit icon onto the StuffIt Expander icon. (Many Macintosh users leave the StuffIt Expander icon, or an alias of it, on the Desktop for just this purpose.
Compression. You can create StuffIt archives using, for example, the shareware Drop-Stuff ...