The APDU (Application Protocol Data Unit) is the communication unit between a reader and a card. The structure of an APDU is defined by the ISO 7816 standards.

There are two categories of APDUs: command APDUs and response APDUs. As the name implies, the former is sent by the reader to the card: it contains a mandatory 5-byte header and from 0 to up to 255 bytes of data. The latter is sent by the card to the reader: it contains a mandatory 2-byte status word and from 0 to up to 256 bytes of data.