At many small colleges, class size averages around 15-20 students, so your classes often consist of a discussion between the professor and the students. You are expected to play an active rather than a passive role. Professors often determine a portion of the course grade based on the quality of your class participation.
In large colleges or universities, it is more common to have large classes (60 or more people) conducted in a traditional lecture style. Undergraduate classes at large colleges and universities are often taught by graduate students (known as teaching assistants) rather than by full professors.