Mac OS X 10.6: Compressing files and folders
Compressed files take up less disk space than uncompressed files, so compressing is useful for making backup copies of your data or for sending information over the Internet.
To compress items:
Do any of the following:
Select the item or items you want to compress and choose File > Compress.
Control-click an item and choose Compress from the shortcut menu.
In a Finder window, select one or more items and choose Compress from the Action menu (looks like a gear).
If you compress a single item, the compressed file has the name of the original item with the .zip extension. If you compress multiple items at once, the compressed file is called Archive.zip.
To uncompress (expand) an item:
Double-click the .zip file.