APDU is defined at ISO 7816 and means "Application Protocol Data Units", where two structured string of bytes, a Command APDU (C-APDU) and a Response APDU (R-APDU) controls the communication between the host (like the Desktop computer) and the smart card, where the smart card receives a C-APDU from the host and returns C-APDU.
In the C-APDU, the first four bytes are mandatory, they are:
CLA -...
