Short for Internet Protocol, and pronounced as separate letters, IP specifies the format of packets, also called datagrams, and the addressing scheme. Most networks combine IP with a higher-level protocol called Transmission Control Protocol (TCP), merged into TCP/IP, which establishes a virtual connection between a destination and a source. While TCP can be used, UDP can also be used in a UDP/IP configuration.
IP by itself is something like the postal system. It allows you to address a package and drop it in the system, but there's no direct link between you and the recipient. TCP/IP, on the other hand, establishes a connection between two hosts so that they can send messages back and forth for a period of time.