The Open Systems Interconnections (OSI) reference model is an industry recognized standard developed by the International Organization for Standardization (ISO) to divide networking functions into seven logical layers to support and encourage (relatively) independent development while providing (relatively) seamless interconnectivity between each layer from different hardware/software environments, platforms, and vendors. There's also a somewhat simpler four-layer Defense Advanced Research Projects Agency (DARPA) model that maps to the OSI model, but the OSI version is the most commonly referred to. I'll reference both models when discussing the various layers.

The following diagram compares the OSI and DARPA reference models:

Unless you're in the business of writing protocols, there's no need to study any of the seven layers in great depth, but it is helpful to understand them conceptually because these layers are referred to by the industry and your IT peers...