Packet switching involves the division of a lengthy message into discrete protocol data units (PDUs). The most generic term for a PDU is packet. Each packet is forwarded separately over the network. The physical pathway for transmission may vary from packet to packet. The pathway for each packet is determined by the examination of the destination address carried in the network layer header of the packet.
Packet switching can be contrasted with circuit switching. In circuit switching, addressing (such as a phone number) is only used to set up a physical pathway. Beyond that initial circuit set up, the message itself is free of addressing. In packet switching, the destination address is carried on each packet. This means that in the event of circuit disruption in the midst of a transmission, individual packets can seek alternate routes to the destination.