IP

IP stands for Internet Protocol. It's the protocol the Internet was built on.

IP allows large, geographically diverse networks of computers to communicate with each other quickly and economically over a variety of physical links. An Internet Protocol Address (IP address) is the numerical address by which a location in the Internet is identified. Computers on the Internet use IP addresses to route traffic and establish connections among themselves; people generally use the human-friendly names made possible by the Domain Name System. [source: ICANN (cache).

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? natively.

IPv4 addresses look like:

192.0.34.163 

IPv6 addresses look like:

[fe80::230:65ff:fe6c:fa88] 

Higher level protocols like: TCP, UDP, RTP, RTSP et etc etc, all use IP.
IP has 4 different methods it can be used in:
unicast?
broadcast?
multicast
anycast? (IPv6 only)

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.

See also:
IPv4 (cache)
website (cache) IPv6 basic (cache) IPv6 advanced (cache)