The OSI model consists of seven layers. The communication functions are broken down into a hierarchical set of layers. Each layer performs a related subset of the functions required to communicate with another system. It relies on the next lower layer to perform more primitive functions and to conceal the details of those functions. It provides services to the next higher layer. The layers are defined in such a manner so that changes in one layer do not require changes in the other layers. By partitioning the communication functions into layers, the problem at hand is much more manageable.
Example of usage – “When communication is desired among computers from different vendors, the software development effort can be very difficult. Different vendors use different data formats and data exchange protocols that do not allow computers to communicate with one another. Recognizing the problem, the International Organization for Standardization (ISO) develops a communications architecture known as the Open System Interconnection (OSI) model that defines standards for linking heterogeneous computers.”