In addition to extra classes for regular students, or continuing education courses for professionals, some universities also offer special non-credit courses in the summer, that is, courses that don't count toward a university degree.
Foreign students often take intensive ESL classes. Other special courses may be offered just for people to take out of interest, for their own enrichment, for groups such as gifted high school students, or retirees or older people. These visitors may stay in dormitories, eat meals, and attend classes together, but they are not enrolled as students of the university, so they may not have other privileges such as library access or health care. Also, a key point for international students: these visitors normally aren't eligible for a longer-term visa, only one for the duration of the course itself.
So depending on context, different universities may have different options.