Basically, there are two types of internships:

Undergraduate Internships: Usually done with bigger organizations that select undergraduates to participate in the internship. It often takes place during the holidays and the average duration is a few weeks. In most cases the intern will receive some sort of stipend or salary and this type of internship may lead to a job offer once the intern is finished with studies.
Graduate Internship: This type of internship could be paid or unpaid and it is to gain practical work experience after the qualification has been completed and in a job related to the qualification. It could be from a few weeks or even one or two years long. This type of internship could also lead to permanent employment.

Below are the latest internship opportunities in South Africa: