Devices that always spin at the same rate are called CAV (Constant Angular Velocity) drives.
Devices that maintain a fixed linear velocity are called CLV (Constant Linear Velocity) drives.
Devices that switch from CAV to CLV when the maximum speed is reached are called PCAV (Partial Constant Angular Velocity) drives.
Most of the recent high-speed CD-ROM drives are PCAV.
Devices that are CLV, but use different speeds on different parts ("zones") of the disc, are called ZCLV.
Most CD recorders use CLV while writing, but some (e.g. 20x and higher) use PCAV or ZCLV.

The following graphs are examples of each type of writing speeds

CLV (Constant Linear Velocity)




CAV (Constant Angular Velocity)




ZCLV (Zone Constant Linear Velocity)




PCAV (Partial Constant Angular Velocity)

From the above graphs you can see the different strategies in the various burning techniques.
The green line represents the burning speed whilst the yellow line is the rotational speed or RPM.
Some of the graphs have a number of dips in them and this is where the Active OPC (Active Optimized Power Control) kicks in.
Active OPC monitors writing power and reflection of the media in use, calculating the optimum laser power and adjusting it in real-time.
Which should in theory result in better quality burning.