Cooperative education or "co-op" is a program spearheaded (though not invented) by Northeastern University as a practical alternative to internships.
In the generally accepted nomenclature, a co-op differs from an internship in that a co-op is paid a nominal market wage and typically will hold an actual titled (usually junior) position at a target employer, meant to become familiar with common tasks related to an industry. In contrast an internship is a non-paid and often non-titled (other than "intern") work experience, meant to become familiar with an industry in a secondary fashion. In practice, the distinction is not always this great; for example, internships sometimes do get paid and do take on basic "real" work.
The cooperative education model was developed originally by the University of Cincinnati. However, Northeastern is home to the National Commission for Cooperative Education, founded in 1962.
At Northeastern, co-op is an optional but highly recommended part of the curriculum. The university schedule and course offerings are arranged to accomodate cycles of students working on co-op or attending classes, in two alternating patterns ("A" and "B") which themselves alternate between pairs of two-quarter and one-quarter periods.
The Co-op department is responsible for preparing new students for their co-op stints, including job searching, resume and reference development, interview skills, etc. The department maintains its own job listing of established co-op positions, and also has close ties with the Career Services department, which can sometimes find additional positions. Students are also welcome to seek out their own co-op positions, as long as the employer agrees to comply with the program's requirements. Beyond that, there is (or was) the "experiential co-op", which is a term usually used for excursions taken for the majority of the co-op period that are deemed to have some rewarding professional value.
The inclusion of co-op into the curriculum means that most undergraduates take a minimum of 5 years to matriculate. Opting out of co-op can lower this period to 4 years or less. Attending year-round without co-op can lead to a completed bachelor's degree in as litte as 3 years, depending on major. However, these alternatives require more schedule planning, as regular course offerings are often arranged based on the co-op cycles; although students can arrange a directed-study course with a lecturer when a desired course is not available or causes unavoidable schedule conflicts.