Register: a register is a very small memory block (for economical reasons their size is rarely greater than a few tens of bits). Being located inside the core of the micro-controller, it is a very fast memory. It is at the top of the memory hierarchy.

Pin: a pin is an electrical conductor that allows to plug/bond an electrical component to a card.

Port: a port allows to an information system to send/receive information. In this sense, we indicate it as  “logical port”.