To build networks, those who designed them started with the idea of separating networking functions in to discrete layers. Each layer performs a specific function and is transparent to the layer above it and the layer below it.
The TCP/IP Model of networking is a different way of looking at networking. Because the model was developed to describe TCP/IP, it is the closest model of the Internet, which uses TCP/IP.
All application data, whether it is an e-mail, a file, an instant message, a video or voice call, is chopped into data segments and encapsulated in Transport Layer PDU's (TCP or UDP segments). The Transport Layer PDU's are then encapsulated in Internet Layer's Internet Protocol packets. The Internet Protocol packets are then chopped into frames at the Network Access layer and transmitted across the physial media (copper wires, fiber optic cables or the air) to the next station in the network.
The OSI Model uses seven layers, and differs quite a bit from the TCP/IP model. The TCP/IP model does a better job of representing how TCP/IP works in a network, but the OSI Model is still the networking model most technical people refer to during troubleshooting or network architecture discussions.
We're going to teach you the TCP/IP model from the top down beginning with the Application Layer.