The OSI Model
The Open Systems Interconnect model is a standard way of understanding the conceptual layers of network communication.
This is a model, nobody builds systems like this.
Each level provided certain functions and guarantees, and communicates with the same level on remote notes.
A message is generated at the highest level, and is passed down the levels, encapsulated by lower levels, until it is sent over the wire.
On the destination, it makes its way up the layers,until the high-level msg reaches its high-level destination.