From wikipedia: good def of packet.
A good analogy is to consider a packet to be like a letter: the header is like the envelope, and the data area is whatever the person puts inside the envelope. A difference, however, is that some networks can break a larger packet into smaller packets when necessary (note that these smaller data elements are still formatted as packets).
Packets are used by the wire protocols, IP. BGP can be looked at as an application that is running on the router, it is actually defined as residing in layer 7 of the OSI network model. It will send information via messages, the messages will be make it down the network stack and be sent as IP packets to other router. As an application BGP will send messages, Layer 3 of the OSI module is defined to send packets.
