BGP stands for a Border Gateway Protocol. The lastest BGP version is 4. It is referred as BGP-4. BGP-4 is one of the Exterior Gateway Protocols and de-fact standard of Inter Domain routing protocol. BGP-4 is described in RFC1771, A Border Gateway Protocol 4 (BGP-4).
Many extensions have been added to RFC1771. RFC2858, Multiprotocol Extensions for BGP-4 provides multiprotocol support to BGP-4.