The BGP4 Protocol

Used (mostly) as an exterior gateway protocol. Probably the most commonly used routing protocol between autonomous systems today.

The data exchanging entities do not have to be routers - but often are.

Instead of advertising routes to networks, the protocol advertises routes to IP prefixes (i.e. BGP4 is a classless routing protocol). An IP prefix contains a length (in bits) and a network address. 

Each route advertised contains a sequence (or set) of autonomous systems it crosses to reach the destined IP prefix.