Rear shocks come in two forms, air or coil. Air rear suspension units are the norm on xc, all mountain and a lot of enduro bikes, an air rear shock is a lot lighter than a coil shock and can be easily tuned to the rider by changing the pressure in the air chamber with a shock pump. Coil shocks are used predominantly on gravity enduro, freeride or downhill bikes. Shock sizes vary with different eye to eye (i2i) lengths and stroke lengths depending on the linkage and travel of the frame. If you don't know what size you need just ask. Most rear shocks are adjustable with rebound, compression and a pedaling platform such as the Fox ProPedal.