Before delving into the mechanics of how information is transferred between computers, you must grow familiar with the terminology used to describe the transmitted data. Many of the layers of the OSI Reference Model use their own specific terms to describe data transferred back and forth. As this information is passed from higher to lower layers, each layer adds information to the original data typically a header and possibly a trailer. This process is called encapsulation. Generically speaking, the term protocol data unit (PDU) is used to describe data and its overhead. Table as below describes the terms used at the various layers of the OSI Reference Model. For instance, as data is passed from the session layer to the transport layer, the transport layer encapsulates the data PDU in a transport layer segment. For TCP and UDP in the TCP/IP protocol stack, the transport layer only adds a header.

As the PDU information is passed down, each layer adds its own header and, possibly, trailer.