For most universities in the US, there are two semesters, fall semester (sept-dec) and spring semester (jan-may). Exams happen at the end of each semester, not in a separate time period.

Some universities have trimesters (3 class periods per year) or quarters (4). But the semester system is most common.

Most universities in the US (90% according to this website according to), are on the semester system in which there are two academic terms, fall semester (sept-dec) and spring semester (jan-may). In this semester system, exams happen at the end of each semester, not in a separate time period.

Some universities have a quarter system, where each quarter is identified with a season and students typically have the summer quarter off. Still other universities have a trimester system which "divides the academic year into three terms of 14 to 16 weeks each, with attendance required in two trimesters per year, to total 28 to 32 weeks of instruction" (wikipedia).