Macintosh, or "Mac" for short, is a general term that refers to the family of computers developed by Apple.
The term first rose to popularity with the Apple Macintosh computer, released in 1984. The Apple Macintosh, or Macintosh 128K, was one of the first computers to feature a graphical user interface (GUI).
With the success of the Apple Macintosh, the company continued to use the Mac brand name. Later computers like the Macintosh II and Macintosh Portable used the name. And the main operating system became known as Mac OS.
These days, the term Macintosh or Mac refers to devices like the Macbook Air, Pro, and iMac, which all run macOS.
Though Apple computers are personal computers, "Mac" is often used to describe Apple computers running macOS. In this context, computers running Windows are referred to as PCs.
Check out the old Mac vs. PC ads for a good example of this branding in action.