The TCP/IP protocol suite is made of five LAYERS.

• Application layer
• Transport layer
• Internet layer
• Network access layer
• Physical layer

The first four layers provide physical standards, network interface internetworking, and transport functions that correspond to the first four layers of t OSI model. The three topmost layers in OSI model, however, are represented TCP/IP by a single layer called the application layer  

 Comparison of the OSI and TCP/IP.

The application layer contains the programming required to support the user's application. Different modules are required for each application such as e-mail or file transfer. The transport layer is responsible for providing reliable communications, including error checking procedures. The internet layer uses the IP protocol to route data between networks, when necessary. The network access layer handles the connection between the end system and the network to which it is attached. The Physical layer specifies the physical (connectors, plugs, adapters) and electrical (voltage and currents) interface between the data communication devices and the network, and this varies by the type of the network.
A number of applications have been standardized to operate on top of TCP. We mention three of the most common here.
1 Simple Mail Transfer Protocol (SMTP)
Simple Mail Transfer Protocol (SMTP) provides a basic electronic mail facility. It provides a mechanism for transferring messages among separate hosts. Features of SMTP include mailing lists, return receipts and forwarding. Once a message is created, SMTP accepts the message and makes use of TCP to send it to an SMTP module on another host. The target SMTP module will make use of local electronic mail package to store the incoming message in the user's mailbox.
.2 File Transfer Protocol (FTP)
File Transfer Protocol (FTP) is used to send files from one system to another under user command. Both text and binary files are accommodated, and the protocol provides features for controlling user access. When a user wishes to engage in file transfer, FTP sets up a TCP connection to the target system for the exchange of control messages. This connection allows user ID and password to be transmitted, and allows the user to specify the file and file action desired. Once a file transfer is approved, a second TCP connection is set up for the data transfer. The file is transferred over the data connection, without the overhead of any headers of control information at the application level. When the transfer is complete, the control connection is used to signal the completion and to accept new file transfer commands.
3 TELNET (Terminal Network)

The main task of the Internet and its TCP/IP protocol suite is to provide services for users. For example, users want to be able to run different application programs at a remote site and create results that can be transferred to their local site. One way to satisfy these demands is to create different client-server application programs for each desired service. Programs such as file transfer programs (FTP), e-mail (SMTP), and so on are already available. But it would be impossible to write a specific client-server program for each demand.
The better solution is a general purpose client-server program that lets users access any application program on a remote computer; in other words, it allows the user to log on to a remote computer. After logging on, a user can use the services available on the remote computer and transfer the results back to the local computer.

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