Border Gateway Protocol (BGP) is a routing protocol which is used to exchange the routing information for the Internet .

It refers to a gateway protocol that enables the internet to exchange routing information between autonomous systems (AS)

It helps to manages how packets get routed from network to network through the exchange of routing and reachability information among edge routers.

It makes the best path selection based on current reachability ,hop-count ,and other path selection characteristics . 

BGP is based on TCP/IP which operates on OSI Transport layer  (Layer 4) to control the Network Layer (Layer 3) .