If your talking about the ScanGaugeII specifically, it can read many of the sensors that your ECM is reading. More sensors than the gauges are showing, and it will give you real numbers such as, your coolant temp is 198F, your intake air temp is 115F, your throttle position is 48%, etc...
It will let you read 'generic' OBDII codes that your ECM may set and all allow you to reset some/most of them.
The ScanGaugeII has a feature to let you calculate your mpg, and other interesting characteristics of a vehicle.
Check out their website and it will give you all the details. ScanGaugeII - Trip Computers + Digital Gauges + Scan Tool
It can be used on any OBDII vehicle. Basically any US vehicle built from 1996 up.
Note : it doesn't read all sensors in all vehicles since each manufacturer's ECM's are programmed differently.