The model rocket is based on the principle of solid rocket propulsion. Solid rocket motors are used inside the body tube for burn out. Such is not the case with launch vehicles, at least partially. Launch vehicles usually use liquid propulsion for burn off, and sometimes hybrid, or solid rocket boosters are used to improve the efficiency of the vehicle flight.
Solid rocket motor
[Source: engineering.purdue.edu]
In model rockets, motors of different grades are used for attaining different levels of altitudes. These grades are based on their specific impulses, and are named by the same logic. Thus, motors of grade A-E when used on rockets get classified under low power rocketry. When motors of grade E-G are used, it is considered as mid power rocketry. All the motors of higher grade are classified under high power rocketry, which requires special skills and permissions to be practiced.
Motors used in model rockets
[Source: wikiwand.com]
Model rockets are equipped with engine mounts within which the solid rocket motors are fixed. Rocket igniters are pushed in through the holes in the engine mount, which are connected to the launch controller. The ignition of the rocket igniters leads to a spark which causes the propellant to burn and the rocket to propel towards the skies!
Motor with rocket igniter