A colleague is someone you work with at your job. When you are a teacher, the other teachers are your colleagues. When you work as a cashier at 7-11, the guy at the deli counter is your colleague as well.
Your colleagues are usually people at the same level or rank as you are. You would not usually consider your boss to be a colleague. This noun is from French collègue, from Latin collega "a person chosen along with another," from the prefix com- "with" plus legare "to appoint as a deputy."