In the US, a course is measured in terms of credit hours. Every degree will have a certain number of credit hour requirement that needs to be fulfilled for graduation. For example, a MS degree might require 36 credit hours. So, the student needs to take enough number of courses that will sum up to 36 hours. Usually, one course translates to 3 credit hours. But there are courses that are 1 credit hour or 4 credit hours. For example, a student working on a thesis during his/her MS degree might be allowed to take 6 credit hours worth of research. So, however you look at it, fulfilling the credit hour requirement is mandatory. A student works with his/her advisor to figure out the right courses that are needed to satisfy this requirement and work towards a plan of action.