With some schools calling their programs “co-ops” and others calling their programs “internships”, spotting the difference can be hard.. 
Generally, the major difference between most co-op and internships programs is that co-ops usually consist of alternating academic programs and paid work terms. Throughout the course of their time in school, a student might take three or more co-op placements at different companies or organizations, with the expectations for the student rising at each subsequent placement.
Internships are usually a “one-and-done” model, with a student doing one internship during their time in school. An internship might last 12 to 16 months, during which they become more comfortable with the work and take on more responsibilities.
That being said, there are a few co-op programs that function more like internships: students take multiple work terms back-to-back with the same organization. In Canada, a not-for-profit organization,  Co-operative Education and Work-Integrated Learning Canada (CEWIL) accredits co-op programs that meet their criteria.