What is a spin? Very simply a spin occurs when an aircraft in a stall also starts to yaw, and when that happens the aircraft descends rapidly with the wings stalled, and at the same time it is yawing to one side or the other and consequently turning, sometimes quite rapidly and pointing towards the ground which starts to come towards you quite rapidly.
Spins are rare, however, one can be a rude or even fatal awakening should you have not trained for the recovery from one.
Spin training is not done as part of training nowadays, but it is still useful to learn what happens if you go past the incipient spin stage and you do need to take recovery action. Standard recovery is to put in full opposite rudder until the rotation stops, relax the back pressure, or push the stick forward to unstall the wings, then ease out of the dive keeping the wings level. If power was on during the entry, reduce power at the beginning of the recovery action, and then add it again once the nose is above the horizon to regain height.
It is possible to spin the Cessna 172, and once in the spin it really does spin quite well. Recovery is easy. Watch this video and you will see what I mean. It is necessary to enter with about 2000 RPM and stall it quite sharply and as it is about to stall, put in full left rudder, it does not spin to the right very easily.