How do upper and lower division classes relate to your major and minor?

Answer:

Lower division classes are broader "basic" courses commonly in the form of prerequisites for a major or minor. Passing these classes is typically required to be permitted to enroll into upper division classes, which are more specific to a major or minor.

For example, a Psychology 1 course at UC Berkeley is a lower division class that is required for majoring in Psychology, with its focus on the general basics of psychology. Passing Psychology 1 is required for enrollment into an upper division Psychology course such as Psychology 130 (Clinical Psychology), in which the material is much more focused and specific.
Lower division courses tend to be more introductory and broadly survey a subject. They give vocabulary, nomenclature, history, background and fundamentals.

Upper division courses will delve much deeper into specific and limited aspects of a subject. These courses will assume the student already has the vocabulary, nomenclature, history, and background to understand the course material. For this reason, most upper division courses will have a prerequisite of one or more related lower division courses.

Some subjects, such as mathematics, are building block in nature and success in upper division courses will require skills and knowledge acquired in lower division courses.