Apple is the name of the company (Apple, Inc.), which has been around about 35 years. Their first product was the Apple Computer (the Apple 1)
About 10 years later, Apple introduced the Macintosh computer, which was originally a small beige all-in-one box, with a mono 9-inch screen.
They've kept that Macintosh name, or Mac.
The term Apple could refer to any product sold with Apple's name on it.
Mac or Macintosh really only refers to products sold with that name, including computers, and operating system. (Mac OS X, or Power Mac G4, etc)
Of course, you may be asking for the difference between a baked apple, and mac 'n cheese. Well, that's not interchangeable either!