Schools become accredited in the United States by first requesting to be evaluated by an independent accrediting agency. Then, they must pass the review to earn accreditation.
Schools don't have to earn accreditation as the process is completely voluntary and nongovernmental. After an independent accrediting agency is chosen, the agency looks at and evaluates the school's programs. Not only does each agency have their own standards, but they also have varying levels of prestige attached to them. There are two types of accrediting agencies: recognized and unrecognized. Recognized agencies are approved by the U.S. Department of Education, while unrecognized agencies have not had their standards reviewed by the Department.
Before an agency comes in, the school has typically familiarized itself with the agency's standards as well as prepared for several months for the review. To pass a review is an endorsement of the agency that the school meets the standards of an institution of higher learning. Schools may then include that endorsement in the information they give out to potential students.
Accrediting agencies are private educational associations. Due to this, many agencies specialize in certain fields of study, such as medical- or engineering-focused agencies. Additionally, schools can choose to have multiple accreditations covering more than one program.