I'm pretty sure they're just an OBD II module. All cars after a certain date ('96, I think) have that connector under the dash somewhere.
I bought a couple modules from Gearbest a couple of years ago and used an android app called Torque to monitor more detailed car data on 3 different vehicles.
I imagine the app takes it up a level and correlates the data to some elementary maintenance procedures.
Some insurance companies use them to monitor your driving habits (mph mostly) to set insurance rates.