IPv4: Internet Protocol version four, or IPv4, is a system of addresses used to identify devices on a network. Originally described in 1981 in RFC791, IPv4 is the most widely used Internet layer protocol, and at this point is used by the vast majority of users to connect to the Internet.
IPv6: IPv6 is short for "Internet Protocol Version 6". IPv6 is the "next generation" protocol designed by the IETF to replace the current version Internet Protocol, IP Version 4 ("IPv4").