Border Gateway Protocol (BGP) is an inter-AS protocol, meaning that it can be deployed within and between Autonomous Systems (AS). An Autonomous System is a set of routers under a single technical administration.
An Autonomous System uses an interior gateway protocol and common metrics to route packets within an Autonomous System; it uses an exterior routing protocol to route packets to other Autonomous Systems.
BGP sends update messages that consist of network number-AS path pairs. The AS path contains the string of Autonomous Systems, through which the specified network can be reached. An AS path has some structure in order to represent the results of aggregating dissimilar routes. These update messages are sent over TCP transport mechanism to ensure reliable delivery. BGP contrasts with IGPs, which build their own reliability on top of a datagram service.