An OBD II scanner is a simple car diagnostic tools that is used to read and clear Diagnostic Trouble Codes (DTC).
These devices are designed to interface with the various modules (ECU, TCU, ABS, Airbag, Traction etc) via the port located under the dashboard of the vehicle.
These scanners can only read and clear codes in the Engine Module (ECU). They are not able to diagnose other modules such as ABS, Airbag, Transmission, Climate Control, Traction Control, etc.
These are scanners that your local auto parts store uses. They will not show any codes if your ABS light is on, airbag/SRS light is on, the transmission is acting up.
These scanners are able to read and clear codes from various modules such as Engine, Transmission, ABS, Airbag with some of them being able to diagnose all systems on your car.