What are the differences between solid and liquid rocket motors?

Solid rocket motors use pre-mixed solid propellant (fuel and oxidizer combined) that burns from a shaped grain surface. They are simple, storable, and reliable but cannot be throttled or shut down once ignited. Liquid rockets pump separate fuel and oxidizer into a combustion chamber, allowing throttling, shutdown, and restart, with higher Isp (300-450s vs 250-290s for solids). However, liquid systems are more complex with turbopumps, valves, and cryogenic handling requirements. Solids are used for boosters and missiles; liquids for main engines and spacecraft propulsion.