Data loggers (data acquisition systems) let you record what your engine and/or vehicle is doing out on the race track.  The generally consist of a "brain box" which controls the process and stores the data, sensors which convert pressures, temperatures, shock position, etc into voltage signals which the "brain box" can then store in its memory, some method of getting the data out of the box into a PC (laptop or desktop) so you can then graph or make charts and reports of what happened.  

The features in the PC software is generally what determines how much you can do with your data:

This goes to show you that the power of the software which analyzes the data is key to understanding what your car and engine are doing.

NOTE:  We also have our DataMite Mini USB which can be used for flow bench testing, cam measurements, valve spring testing, suspension spring (coil or roll bar) testing, weather station, and shock dyno testing.  AND, we have our Micro Logger used for our Quick Cam Checker and Blowby Recorder.