Earning a degree requires a certain number of credits earned from classes. A typical 4-year undergraduate degree is 128 credits, spread over eight, 16-credit academic semesters. The academic catalogue is a useful resource for information on what a credit is, or for particular classes or major requirements.
A typical class is 4 credits, and four such courses are typically taken in a semester, or two during a summer half-semester. Labs, seminars, and recitations commonly grant only 1 credit, and may be required to take concurrently with the corresponding lecture class.
Taking 3 or more classes during a Fall or Spring semester (12 credits) is the minimum to be considered a full-time student. Unfortunately, this also means taking 3 classes costs the same tuition as 4, so it is not recommended. When taking 1 or 2 classes during a semester or the summer, you are charged only on a per-class basis. Most 1-credit labs, seminars, etc. have no additional tuition cost.