What do thrusters work?

I'm curious, could you explain to me in simple terms how thrusters work? I've heard they're used in spacecraft and other vehicles to generate thrust, but I'm not entirely sure how the process works. Are there different types of thrusters, and if so, how do they differ from each other? Additionally, what are some of the key factors that influence the effectiveness of thrusters, and how do engineers optimize their designs to achieve maximum performance?