OSI model, Open Systems Interconnection model

A model that standardizes communication of a computing system to provide interoperability using standard communication protocols. Data flow is divided into seven layers, ranging from the actual hardware to what a user sees on a computer screen:

  1. Physical Layer — Transmits raw bit stream over physical medium.
  2. Data Link Layer — Defines the format of data on the network.
  3. Network Layer — Decides which physical path the data will take.
  4. Transport Layer — Transports data using transmission protocols, including TCP and UDP.
  5. Session Layer — Maintains connections and is responsible for controlling ports and sessions
  6. Presentation Layer — Ensures that data is in a usable format and is where data encryption occurs
  7. Application Layer — Human-computer interaction layer, where applications can access network services.

The layers are hierarchical, with each layer serving the next, beginning with the Layer 1, the physical layer. You might get questions about these layers, especially the network layer. Network World has a good explanation of each layer. For more information, check out the Wikipedia article.