For those budding internet geeks out there, IP means Internet Protocol. It is the basic addressing protocol on which higher-level protocols, which actually convey end-user data, are layered.
Current variants routed on the Internet are IPv4 and IPv6. Technically IPv4 is deprecated and IPv6 is the current standard, but in practice both are supported.
IPv1 through v3 were research protocols, not officially designated as such but retrospecively enumerated after IPv4 was developed.
IPv5 was an experimental protocol designed to support better video streaming and known as Internet Stream Protocol and whilst it had benefits it retained the 32-bit addressing scheme of IPv4 and couldn't keep up with the growth of the Internet, so was abandoned in favour of the less-radical but more-extensible IPv6 proposal.