What is accreditation?

Accreditation is both a status and a process.
Accreditation Status
accredited colleges, accredited universities, and accredited schools have voluntarily undergone a formal evaluation process by a impartial trusted professional society, non-governmental body, or government agency and has been granted official approval, credit or recognition for meeting or exceeding specific criteria, requirements or standards.
Accreditation Process
The formal evaluation of an organization according to accepted criteria or standards. Accreditation may be done by a recognized and trusted professional society, non-governmental body, or government agency. 
What is Accreditation for?
Accreditation is meant to serve as an impartial independant verification that institutions are meeting quality standards. (In the context of vocational qualifications) Ensuring that a qualification meets national requirements.
Who Gives Accreditations?
There are several Accreditation agencies. There are six main Regional Accreditation Agencies provide accreditation to their specific area. Though regionally accredited these agencies are national recognized and recognized by either the United States Department of Education (USDE) or the Council for Higher Education Accreditation (CHEA), oftentimes... both.
Do Online Schools receive Accreditation?
Accredited Online Colleges, Accredited Online Universities, and Schools are  often given their official approval, credit or recognition from Distance Education Training Council (DETC). DETC is the Global Leader in Distance Learning Accreditations and is recognized by both the United States Department of Education (USDE) and the Council for Higher Education Accreditation (CHEA). Side Note: Most schools offering both online and offline classes make no distinction between degrees.
What if my school isn’t accredited?
Some employers may not take your degree as seriously as they would had it been from an accredited school. Also, if during your courses you move... you may not be able to transfer your credit hours or degree to an accredited school. 
Is accreditation important?
Honestly each situation is different. In some cases it may not matter, but there are some major advantages. When schools are accredited, they can offer degrees that are accepted just about anywhere. Many regionally accredited schools will not accept transfer credits from non accredited or DETC accredited schools.
Types of Accreditation
Institutional Accreditation
Institutional accreditation normally applies to an entire institution, indicating that each of an institution’s parts is contributing to the achievement of the institution’s objectives, although not necessarily all at the same level of quality.
Specialized Accreditation
Specialized accreditation normally applies to the evaluation of programs, departments, or schools which usually are parts of a total collegiate or other postsecondary institution. The unit accredited may be as large as a college or school within a university or as small as a curriculum within a discipline. 
There are a number of accrediting agencies and associations which are reliable authorities on the quality of training offered by educational institutions.