Generally these are referred to as "labs" or "discussion" sections, as opposed to the "lecture" session.
Giving an example from computer science, it is not uncommon for there to be a lecture session where the instructor (such as the professor) talks about the material, takes questions, etc. Then there is often a lab session, often held in a computer lab, where students can work on hands-on assignments, homework, and course projects. These can be staffed by the instructor/professor or by student teaching assistants, or held as an open lab where the room is reserved but no one conducts the session - students come and go and work as they please.
In fields of communication, philosophy, and history, it is common that this lab session is replaced by a discussion session. The lecture is often of a mass variety, where the professor gives the talk to hundreds of students at a time. During the discussion session the professor or a student teaching assistant holds discussions, readings, gives out assignments, and various other similar tasks.
In physics and biology, again there is often a lecture or mass lecture held by the professor, then sometimes both lab and discussion sessions may be held. Again the staffing and locations vary, but the general theme is the same.
I have personally experienced these in a number of institutions in the US in the fields of biology, chemistry, physics, philosophy, history, communications, art, computer science, and psychology...so it certainly seems to be a very common pattern.
These non-lecture sections are almost always of a less-populated variety as well. If the class only has 20-30 students total, then the lab sections are of the same size. If the class is over 30, I have usually experienced lab and discussion sessions to be smaller, with as little as 12-20 students maximum - but this usually varies by room and lab availability and subject, and thus will vary by University, department, and subject.