Data Encapsulation
Data encapsulation describes how data is sent from the source to the destination. This discussion is usually done with reference to the OSI reference model.
Communications originate at the application layer and travel down the OSI stack to the physical layer.
As the data makes its way down the OSI stack, each layer adds a PDU (protocol data unit) (aka header) to the data.
This PDU is specific information meant for the same layer on the receiving computer.
The physical layer is the only layer in which direct communication takes place.