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Here are a few simple definitions.
They all apply to either the front or rear suspension.
SAG
Is usually measured in millimetres and is the difference between the length of the suspension fully extended, and when it is compressed.
There are two types of SAG
Free SAG. This is measured with the suspension compressed by the weight of the bike only.
Rider SAG. This is measured with the suspension compressed by the weight of the rider and the bike.
Pre-Load
When Pre-Load is increased this is the act of making the spring shorter.
When Pre-Load is reduced this is the act of making the spring longer.
Pre-Load is manipulated to achieve the required SAG.
When installing a spring away from the bike a recommended pre-load will be given, usually in millimetres, which is the distance the spring needs to be shortened from its free length.
New bikes or after market companys will supply suspension with pre-load that is based on either an educated guess or a scientific guess.
Upon delivery a bike should always be set-up precisely for the regular rider.
Compression Damping
Compression damping controls how the suspension is compressed when the bike hits a bump. It also controls how the front is compressed under braking and how the rear is compressed under acceleration.
Compression damping may have High and Low speed adjustment.
They both refer to the speed of deflection caused to the wheel by a bump, not the speed of the motorbike. Low speed compression damping is the one that provides feel.
Rebound Damping
Rebound damping controls how the suspension recovers from being compressed, either from bumps, braking or acceleration.
Compression and rebound damping are controlled by the flow of oil through valves within the suspension.
PRE-LOAD, SPRING RATE, COMPRESSION DAMPING AND REBOUND DAMPING, ALL INTER REACT WITH EACH OTHER AND SO IF THEY ARE NOT ALL IN BALANCE THE HANDLING OF YOUR BIKE WILL BE AFFECTED NEGATIVELY
Hope this helps,
John