A "kouhai" is your junior at work, school, or a group. Usually they belong to the same organization as you but they entered the organization after you. A "kouhai" could also be someone who has entered the same "industry" as you but at a later date. In this case "industry" has a very broad meaning, e.g. the entertainment industry, a professional sports league.
A sempai with a kouhai, an elder student with a younger student. Senior (Sempai/Senpai) with a Sophmore (Kouhai).