* 수업 (sueop): This is the most common word for "class" referring to a lesson or a period of instruction in a school or other educational setting. This is probably what you're looking for in most situations.
* 강의 (gangui): This word refers to a lecture or a formal class, often in a university setting.
* 반 (ban): This word means "class" as in a group of students in a specific course. For example, "1반" (il ban) means "class 1."
So, depending on the context, you'd use one of these words. `수업 (sueop)` is the safest bet for general use.