Layer 7, application: The application layer provides networking services to a
user or application. For example, when an e-mail is sent, the application
layer begins the process of taking the data from the e-mail program and
preparing it to be put onto a network, progressing through Layers 6
through 1.
The combination of the seven layers is often called a stack. A transmitting
workstation traverses the stack from Layer 7 through Layer 1, converting the
application data into network signals. The receiving workstation traverses the
stack in the opposite direction: from Layer 1 to Layer 7. It converts the
received transmission back into a chunk of data for the running application.
When the OSI model was created, there was an industry initiative that tried to
implement a universal set of OSI network protocols, but it was not adopted.
Most popular protocols today generally use design principles that are similar
to and compatible with the OSI model, but they deviate from it in some areas
for various technical reasons. That said, the OSI model is still considered the
basis of all network communication.