IP stands for Internet Protocol. It's the protocol the Internet was built on.
The set of rules that govern how devices communicate over the Internet. The Internet Protocol specifies the format of the packets that devices use to transmit messages through the network. It also specifies the addressing scheme that routers use to transmit messages to their destinations.
IPv4 is the original implementation. Gradually everyone is trying to go to IPv6 which has way more potential addresses. With this new version every electronic device in the world is supposed to be able to get an IP address assigned. IPv6 supports techniques like multicast, anycast, DHCP and IPsec (encryption for packets) natively.
IPv4 addresses look like:
IPv6 addresses look like:
There are several more sending paradigms (not implemented in the IP standards?). These include manycast (multicast/anycast mix), groupcast, somecast (realtime adaptive reliable multicasting). The freaks can google them.