The Internet Protocol (IP) is a protocol that delivers a package of bits (a packet or datagram) from a source to a destination on a so called packet-switching network such as the Internet.
The packet is given sufficient information to be transported to the destination. There is no fixed connection between the source and the destination. The network is connection-less. The packet is transported from one node of the network to the other towards the destination. The delivery of the packet is not guaranteed. It is just a best-effort.
IP only transports individual packets through the network. To transport a message, or a large block of data, another protocol has to be used, to divide the data into smaller segments. Most of the time, the TCP protocol is used for that. The TCP protocol numbers the packets and gives them a checksum, thereby ensuring that the total amount of data sent is correctly received. The combination TCP/IP is commonly reffered to when talking about the delivery of data on the Internet.
A general description of IP is given in RFC791.