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.)