Mail gives suggestions based on your contacts and messages on your Mac and devices signed into iCloud. 1 Enter a subject for your message. Write your email in the body of the message. The family of Macintosh operating systems developed by Apple Inc. Includes the graphical user interface-based operating systems it has designed for use with its Macintosh series of personal computers since 1984. The current Mac operating system is macOS, originally named 'Mac OS X' until 2012 and then 'OS X' until 2016.