How to Zip and Unzip Files on Your Mac
Files you download from the Internet are often compressed or zipped so that they take up less space and arrive much faster than files that haven’t been compressed. You can easily identify compressed files by their extensions, such as .zip (a common standard used in OS X and Windows) and .sit. Before you can use these files, you must unzip them.
Apple used to include a program called StuffIt Expander to decompress zipped files, but doesn’t now that OS X lets you unzip files (but not .sit files). However, StuffIt from SmithMicro Software still comes in handy for opening other types of compressed files, notably the .sit or .sitx compressed types. Go to www.stuffit-expander.com or www.stuffit.com/mac/index.html to download a free version of the software or to splurge for the Deluxe version. In addition to compressing files, StuffIt Deluxe lets you encrypt and back up files.
Meanwhile, you can archive or create your own .zip files through OS X, which is useful if you’re e-mailing a number of meaty files to a friend. Right-click (or Ctrl-click) files you want to compress inside Finder and choose Compress Filename. The newly compressed files carry the .zip extension. The archive is created in the same location as the original file and is named originalfilename.zip. You can also choose File→Compress. If you compress a lot of files at once, the archive takes the name Archive.zip.
By default, compressed files are opened with the Archive Utility. It appears in the Dock (in Leopard) while the files are being unsqueezed, unless you choose to open them with Stuffit Expander or some other program.