IP is the Internet Protocol. It is a mechanism by which packets may be routed between computers on a network-of-networks. IP allows computers to be connected using various physical media, ranging from modems to Ethernet cabling, fiber-optic cables and even satellite and radio links.
IP is designed to be robust, and to gracefully handle the loss of some connections. Individual packets of data are routed by hosts with little knowledge of the overall network structure - just a few local routing rules.
As its name implies, the global Internet is constructed using the IP network protocol.