What is a Hybrid Rocket?
A hybrid rocket is a propulsive device that uses both a liquid and a solid substance as their fuel and oxidizer. Most commonly, the oxidizer is liquid and the fuel is solid. Like a solid rocket engine, the fuel is molded into the combustion chamber and the oxidizer is located in a pressurized vessel above it. The oxidizer can then get pumped into the combustion chamber via an injector which combusts with the presence of the solid fuel, enabling the engine to provide thrust to the vehicle attached.
The pros and cons of hybrid engines
Hybrid engines are used mainly within university college teams for the immense simplification it is compared to a liquid engine. Other pros to a hybrid engine are its safety and still having the ability to throttle. A lot of the fuel/oxidizer combinations that work well are inert on their own, meaning that handling outside of the engine is much easier, especially for groups with not a lot of money for fancy equipment (college teams!). This DOES NOT mean that safety is still involved for caring for your products, especially when they are in the engine ready for firing. The other pro is the ability to throttle, which is something a solid rocket booster can not do (at least efficiently!) as once it is lit, it burns through the entire grain.