A co-worker is someone you work with. Your ice cream shop co-worker might scoop the ice cream while you're on milkshake duty.
When you work with someone, that person is your co-worker. The word implies that you work side-by-side — you wouldn't usually refer to your boss as your co-worker. If you work in an office, your co-workers probably have cubicles and computers that are similar to yours, and if you work on a farm your co-workers are right there beside you weeding the beets or watering the tomatoes. The prefix co in co-worker means "together" or "mutually."