Mac is short for Macintosh, a brand of computers created and sold by Apple Inc.. Macintosh products are the desktop and mobile computing family, whereas products such as the iPod, the iPhone or the iPad don't carry a "Mac" name.
Mac OS X
All current Macs ship with Mac OS X ("Pronounced: Mac OS Ten"), and allow you to install Windows or Linux either through virtualization software, using an alternative boot loader, or Apple's provided "Boot Camp" feature to install Windows on your Mac.
Currently produced Macs
macbook-pro about the MacBook Pro, both the newer Unibody models as well as the earlier Intel-based line.
macbook-air about the MacBook Air, a ultra-portable laptop based on flash storage.
imac about the iMac, a desktop computer with a built-in display. This tag compromises all iMacs, from the first "iMac G3" to the latest Unibody iMac.
mac-pro about the Mac Pro, a professional desktop computer with options for high-end components, primarily used in media production environments.
powerbook about the laptop line that was manufactured from 1991 to 2006. The line was discontinued and replaced for the MacBook brand when Apple switched to Intel processors.
macbook for all questions about MacBooks – or specifically the budget MacBook line, as compared to the professional MacBook Pro line. Apple currently does not produce any MacBooks anymore.