BGP is Border Gateway Protocol, used on the internet both inside of and in between networks.
BGP, or Border Gateway Protocol is used primarily on the Internet today in two flavors which have many similarities. eBGP, or External BGP, is used to communicate routing information between different ASs (Autonomous Systems). iBGP, or Internal BGP is used to communicate routing information inside a particular AS.
BGP is defined by RFCs, in particular RFC4271.