Packet

A packet is the fundamental unit of data carriage in modern data networking; is consists of a block of data, the 'payload', and a header, which contains the information needed to get the packet from the source to the destination (network addresses, etc).

A useful analogy can be made to a physical letter, in which the wrapper, the envelope, carries the information needed to get the letter from the sender to the recipient; the actual message is carried inside.