OS X Yosemite: Compress or uncompress 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.
Compress a file or folder: Control-click it or tap it using two fingers, then choose Compress from the shortcut menu.
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.
Uncompress (expand) an item: Double-click the .zip file.