The U.S. Department of Education defines diploma mills as schools with little or no state regulation that provide fraudulent diplomas. These schools may have unrealistically short degree programs or claim you can get your education significantly lower tuition than traditional schools. They may also claim accreditation by fake accrediting agencies, so the best way to avoid falling into a scam is to check the U.S. Board of Education's Accredited Schools list before you begin looking for a school.
While the Internet is the most popular place for diploma mills to market their businesses, there are still many accredited online schools to choose from. According to the Department of Education, accredited schools agree to be reviewed regularly to determine their adherence to standards of coursework, teacher quality, equipment, facilities and supplies. Accreditation exists to help a student get the most out of her hard work and tuition money.
Though the Department of Education does not accredit schools itself, it does provide a list of regional and national accrediting agencies, as well as a list of agencies that specifically grant accreditation to distance-learning programs. See "Resources" for the list. The Board of Education also provides a search by institution, so if you have a specific school in mind, you can check that school's accreditation. Examples of popular accredited online universities include University of Phoenix, DeVry University and Ashford University Online.
The Department of Education recommends checking with the Better Business Bureau or your state attorney general in addition to using their search tool for accreditation.