Co-Op mean while you are studying you can gain practical work experience.
Suppose you have 4 semesters, and you have co-op studies, then most probably out of four semester you will work full time in 1 semester. That work normally provided by colleges/universites in different companies.
Its better to go for co-op as you can gain some money wile working (Salary) and you will gain value experience. Its a sort of articles/internship