Are thrusters push or pull?

Excuse me, could you clarify something for me about thrusters? I've been wondering if they are designed to push or pull. I understand that thrusters are used in a variety of applications, such as spacecraft propulsion and underwater vehicles, but I'm not entirely sure how they work in terms of their fundamental function. Are they essentially designed to push against something to create motion, or do they function by pulling on an object? I'd appreciate your insight on this matter.