Two numbers like 18.XX means that it is intended for non-math majors so 18.02 is the basic multivariable calculus that every MIT alum needs to graduate, 18.022 is multivariable calculus with an emphasis on theory and 18.023 is with an emphasis on application and usually taken by people considering math majors (though also fulfill the core requirement) The class numbering schema is by field: http://math.mit.edu/academics/classes.php

18.1XX Analysis / Calculus

18.2XX Discrete

18.3XX Applied

18.4XX Computational

18.5XX Logic

18.6XX Probability and Statistics

18.7XX Algebra and Number Theory

18.8XX Just project lab

18.9XX Topology and Geometry