OSI Reference Model

The OSI Reference Model is a method of defining and displaying the various interconnections that happen within a network. The layers go up in complexity near the top of the list, and devices and protocols on the same layer will communicate with each other.

For example if you have two switches on a network, then even though they'll be passing all sorts of information back and forth that encompasses all the layers, but the switches themselves will only be concerned with and interpret information on layer-2. While layer-1 information is passed to them, their Network Interface cards will handle that, and while information for higher layers is sent as well, it's contained within the information that the switches are passing around.