What is Time Machine?
Time Machine is automatic backup software that comes with Mac OS X. It allows you to backup your entire Mac, including system files, applications, accounts, preferences, email messages, music, photos, movies, and documents to an external drive. After the initial backup is complete, Time Machine will continue to perform hourly backups on any files that have been changed since the last time it ran. When your external drive is filled, Time Machine will start to delete the oldest existing backups in order to free up space.