The iPhoto Library item is a package—a special kind of Mac OS X folder. If you double-click the iPhoto Library item, your Mac simply starts or switches to iPhoto.
But there is a way to get inside if you must. Control-click the iPhoto Library item and choose Show Package Contents from the shortcut menu. Inside, you’ll find a bevy of folders and files that constitute your library.
Warning: Leave them be. Always use iPhoto to add photos to or remove photos from your library: drag photos into and out of the iPhoto window.