Diploma mills are companies that sell fake degrees. With a professional-looking website and the increased capability of printers nowadays, it's not that hard at all to print up an official-looking diploma at all. Rarely do they use the name of an existing college or university, to avoid a lawsuit or direct attack from law officials. Rather, they make up their own official looking degree names such as Chicago Technical College. Sounds official, huh? Some diploma mills go all the way offering official-looking transcripts and employment verification services.
Some diploma mills will claim that they've found a "legal loophole" or some other way in which you can obtain a degree without doing any type of academic coursework whatsoever. "Get a Bachelor's degree in 1 week. Pay an extra $200 and get a Master's." There are even "legitimate" schools that do offer real coursework, but they are not regionally accredited. They may even claim "national" or "international" accreditation. But why would you want to get a piece of paper that wouldn't be accepted in the professional or academic world? Regional accreditation is the way to go.