A co-op is usually longer and during the school year. An internship is usually shorter and during the summer.
It is usually easier to find a co-op than a summer internship.
You can earn college credit while working by taking the co-op class. During the fall and spring semesters some students need student status for financial aid or insurance reasons. In this case, you will need to enroll in the co-op class or some other online course. During the summer, in most cases, it is up to you on whether or not to seek the co-op credit.