Internet Protocol (IP) is the method by which data is sent from one computer to another via the Internet. Each computer on the Internet has at least one IP address that uniquely identifies it. When you send or receive data (view a web page, send an email), the message (data) gets divided into little chunks known as packets. Each of these packets contains both the sender's Internet address and the receiver's address. Any packet is sent first to a gateway computer that understands a small part of the Internet. The gateway computer reads the destination address and forwards the packet to an adjacent gateway that in turn reads the destination address and so forth across the Internet until one gateway recognises the packet as belonging to a computer within its immediate vicinity (domain). That gateway then forwards the packet directly to the computer whose address is specified.
The Internet Protocol is responsible for delivering the packets and it is the job of another protocol, the Transmission Control Protocol (TCP/IP) to put them back in the right order.