At Berkeley, course numbers >100 are upper-division, and those <100 are lower-division, introductory classes. Especially in the CS department, the upper-division courses are far from introductory. 61B is the the second in the 61A-B-C series, which is required for all CS majors. (A fourth lower-division class, CS 70 ("Discrete Mathematics and Probability"), is also required, but is independent of the 61 series.)

61B is the second beginners course of Berkeley Computer Science (61A being the first). This is essentially a 102 course.