A context diagram is a data flow diagram that only shows the top level, otherwise known as Level 0. At this level, there is only one visible process node that represents the functions of a complete system in regards to how it interacts with external entities.
A typical context diagram is made up of two elements, which are labeled boxes that represents various entities and labeled lines that shows the relationship between them. Information shown in a context diagram can also include classifications of external entities. This classification shows how each entity interacts and influences the system, whether directly or indirectly.