A packet is a block of user data together with necessary address and administration information attached, to allow the network to deliver the data to the correct destination. One data connection will usually carry a stream of packets of data that will not necessarily be all routed the same way over the physical network.
Analogous to a physical packet sent through the post with the address written on the outside, this provides the information the network (the postal service) needs to get the packet to the correct destination.
Packets are routed to their destination through the most expedient route (as determined by some routing algorithm). Not all packets travelling between the same two hosts, even those from a single message, will necessarily follow the same route.
The destination computer reassembles the packets into their appropriate sequence. Packet switching is used to optimise the use of the bandwidth available in a network and to minimise the latency. Ethernet, X.25 and Frame relay are international standard layer 2 packet switching networks.
based on FOLDOC