The scanmaster is an onboard scantool that wires into the ALDL port under the radio and allows real time measurement of a variety of engine parameters. The powerlogger allows you to record data from a run and view it on your laptop later. Both are very handy tools as the scanmaster can help you see sensor outputs like coolant temp and battery voltage, give you an indication of AFR, and most importantly let you know about knock retard. The powerlogger will let you see what the other parameters are doing when you are tuning and help others help you sort out tuning issues you may be struggling with. It will also allow you to log your boost, fuel pressure, and AFR provided you have the other addons to measure these parameters. I believe the manuals are out there for download on vendors sites so you can read more about the features of each.