Most classes are taught in large lecture halls by a professor. Enrollment in a lecture class might exceed 100 or even 200 students who sit and take notes while the professor presents information. Information is presented orally or with the assistance of visual aids and PowerPoint presentations. There isn't usually interaction between the professor and the students during class--questions must be asked during the professor's office hours or presented to the teaching assistant.
One of the most recent developments in American universities is the growth of online offerings. Online courses convey information through a university-based website and require students to share information on that website or via email. Some online courses require students to provide feedback to classmates and take exams online.
A seminar consists of 20 to 30 students in a classroom that might resemble a high school classroom. The professor presents information in a traditional manner, but there is usually an opportunity to ask questions and participate in discussions. Attendance is usually mandatory.
Teaching assistants (TAs) facilitate follow-up courses that expand upon topics covered in lectures. These classes provide time for additional support, discussion and questions. Class size is usually small.
Labs are an accompaniment to a larger lecture or seminar class. It's a place where students apply what they have learned and demonstrate proficiency in particular skills or methods. Labs are often science-based, but can include foreign language and computer coursework as well.