The body or frame
Casting reels have frames made of graphite or aluminum. Spinning reels have bodies made of plastic, graphite or aluminum. Graphite is lighter but will flex. Aluminum is the material of choice for more costly and more durable reels of either style. Whether frame or body, the purpose is the same. It is used to hold the components that make up the fishing reel.
The spinning reel has a spool that lays parallel to the fishing rod. Line is wound on it with a rotating bail that orbits around the spool. Line is peeled off when casting by opening the bail, allowing the weight of the bait to pull line off the spool. Casting reels have a spool that sits perpendicular to the fishing rod. The spool rotates taking up line as the handle is turned. When casting the spool is disengaged, it will spin freely. The weight of the bait spins the spool as it pulls line off.
Automatic braking system
Only casting reels have an automatic braking system. These are set to control the rate of spin of the spool during casting. One of the drawbacks of casting reels is the spool will increase in speed as the line is being pulled off. Sometimes the spool will spin faster than line can be taken off causing a tangle of line called a backlash. This happens most often when the bait hits the surface of the water.
All fishing reels have some type of drag system. This is important when fighting a fish to allow line to be taken off the spool in a controlled manner while giving some resistance. This resistance wears the fish down so it can be reeled in. The drag system is made up of washers of different materials sandwiched together. A knob is used to increase or decrease the pressure on the washers. Drag is increased with an increase in pressure on the washers, making it harder for line to be pulled off the spool.
Spinning reels usually have handles that can be set for either right or left handed operation. The handles on casting reels are fixed at the factory. Most spinning reels have an anti-reverse mechanism that prevents the handle from being turned backward. This can be turned off when preparing to cast so you can position the bail and the line where you need them to be for better casting.
Ball bearing are used in fishing reels to align moving parts and to keep them moving smoothly.