Student work and observation experiences go by a number of different names, including internships, co-ops, practicums, and externships. Sometimes it's hard to tell what an experience should be called—definitions can vary among schools and employers. Following are some general definitions.
Internships are typically one-time work or service experiences related to a students major or career goal. The internship plan generally involves a student working in a professional setting under the supervision and monitoring of practicing professionals.
Internships can be paid or unpaid and the student may or may not receive academic credit for performing the internship.
Cooperative education provides students with multiple periods of work in which the work is related to the student's major or career goal. The typical program plan is for a student to alternate terms of full-time classroom study with terms of full-time, discipline-related employment. Since program participation involves multiple work terms, the typical participant will work three or four work terms, thus gaining a year or more of career-related work experience before graduation.
Virtually all co-op positions are paid and the vast majority involves some form of academic credit.
A practicum is generally a one-time work or service experience done by a student as part of an academic class. Some practicums offer pay, but many don't. Almost all are done for academic credit.
An externship or job shadowing experience allows a student to spend between a day and several weeks observing a professional on the job. Such experiences are unpaid, however some colleges and universities pick up travel and/or living expenses. Externships and job shadowing experiences are generally not done for academic credit.