The Internet protocol suite is the set of protocols that implement the protocol stack on which the Internet runs. It is sometimes called the TCP/IP protocol suite, after two of the many protocols that make up the suite: the Transmission Control Protocol (TCP) and the Internet Protocol (IP), which were the first two defined. The authoritative reference on this subject is RFC 1122, which can be found at http://www.ietf.org/rfc/rfc1122.txt.
The Internet protocol suite can be described by analogy with the OSI model, which describes the layers of a protocol stack, not all of which correspond well with Internet practice. In a protocol stack, each layer solves a set of problems involving the transmission of data, and provides a well-defined service to the higher layers. Higher layers are logically closer to the user and deal with more abstract data, relying on lower layers to translate data into forms that can eventually be physically manipulated.
The Internet model was produced as the solution to a practical engineering problem. The OSI model, on the other hand, was a more theoretical approach, and was also produced at an earlier stage in the evolution of networks. Therefore, the OSI model is easier to understand, but the TCP/IP model is the one in actual use. It is helpful to have an understanding of the OSI model before learning TCP/IP, as the same principles apply, but are easier to understand in the OSI model.