Model rocketry is a hobby simiar to model airplane. In the U.S., there are two distinctly different types of hobbyist.
Amateur rocketry hobbyists experiment with fuels and make their own rocket motors, often launching rockets hundreds of miles into the ocean. Amateur rockets are dangerous because nonprofessional rocket motors often explode, and can be dangerous to fuel.
Model rocketry is different because these hobbyists purchase small professionally-manufactured disposable solid-fuel rocket motors. Since they are professionally designed and constructed, they are far safer. The motors come in standard sizes and powers. The motors have cardboard bodies, and lightweight molded ceramic nozzles. The top of the rocket motor is designed to burn through, and emit a small pulse of hot gas. This is used to initiate recovery (usually, push out a parachute), or ignite the next stage of rocket motor. Motors are electrically ignited with a short length of nichrome wire pushed into the propellant and tamped with cotton or toilet paper. After a motor is used, it is thrown away.
Model rockets are designed to use these standard motors. The rocketeers experiment with rocket sizes, shapes, payloads, multi-stage rockets and recovery methods. Some rocketeers even build models of larger rockets.