In recent decades, the role of community colleges has changed significantly. Once perceived as a place for students "not smart enough" for university, today community colleges are required to train for a wide variety of challenging and technical careers. In the computer field alone, most colleges offer programs in disciplines that didn't exist just 10 years ago. Even auto mechanics now need to work with a range of highly technical computerized equipment. As the programs offered by community colleges become more complex, the need for accountability is increasingly obvious.
The simplest way to describe accreditation is that it's a way for schools to agree on what constitutes a good education. Schools in a given category, geographic region, or educational specialty will typically belong to one or more professional organizations. These organizations establish criteria for things like curriculum, student-teacher ratios, educational outcomes and the success rate of graduating students. Once a standard is established, they'll apply to a government-recognized body such as the National Commission for Certifying Agencies for permission to offer accreditation to their member schools.
Every accrediting body has its own criteria, but the process is generally similar. A college that wants to be accredited will apply for consideration. They will generally be sent a self-evaluation package, detailing the current standards accredited schools must meet. The college reviews its operations for compliance with these criteria, then submits a report and application to the accrediting organization. The organization sends a peer-review team to examine the school and ensure that their self-evaluation was accurate. Assuming the report is favorable, the certifying body will review the school in depth and recommend any changes that might be necessary, after which the school may be accredited.
Accrediting bodies are, themselves, subject to a certification process. The National Commission for Certifying Agencies, for example, grants accreditation privileges to scores of certifying bodies in a wide range of professions. A complete list is available on their website. A comprehensive explanation of the accreditation process may be found on the AdvanceED website. AdvancED is a joint venture of the North Central Association Commission on Accreditation and School Improvement (NCA CASI) and the Southern Association of Colleges and Schools Council on Accreditation and School Improvement (SACS CASI).