The ZIP file format is a data compression and archive format. A ZIP file contains one or more files that have been compressed to reduce file size. ZIP files generally use the file extensions ".zip" or ".ZIP".
Some files are compressed and decompressed with the ZIP format automatically via email. Mac OS X includes the ability to manually ZIP files. Click once to highlight the file or folder in the Finder. Then select from the menu bar: File > Create Archive. Alternatively, the file can be highlighted before control-clicking (or right-clicking) it to reveal a contextual menu that says Create Archive (of filename). A ZIP archive can be unpacked on Mac OS X by double clicking it. Note that versions of Mac OS prior to 10.3 stored essential file data within resource forks that are not preserved in the ZIP format. Hence, files created within Mac OS 9 through Mac OS X 10.2 that require resource forks need a different compression application like Allume Systems Stuffit.
A number of ZIP archiving and decompression applications are available for the Windows PC and Mac OS X, including StuffIt. Some are free and others require a fee. The advantages using ZIP are faster/smaller file uploads and downloads along with ability to include multiple items within one archive.