IP stands for Internet Protocol. IP is used for software addressing of computers and works at the data link layer. RFC 791 defines IP.
Internet Protocol (IP) supports network connectivity at the network layer of the OSI model. UDP and TCP data packets are encapsulated into IP data packets to be carried from one host to another. IP is a connection-less unreliable protocol providing no guarantee that the data will reach the intended host. When datagrams are delivered they may be damaged, out of order, or the datagrams may not arrive at all. The layers above IP in the OSI network model such as TCP are responsible for being sure correct data is delivered. IP provides supports the addressing and delivery of datagrams between hosts and supports re-assembly of datagrams.
The page about Internet Protocol provides more information about IP including a description of the IP message format.