Block Diagram is a diagram showing a general description of a system and its functions. It shows the major components of a system and the interconnections of these component in block form, and each block is labeled for identification purposes. This type of diagram is generally used in conjunction with text material.
The above picture shows an illustration of an automobile’s electrical power, starting, and ignition systems. The purpose of a block diagram is assisting in interpreting a diagram. In this case, it explains the functions of the automobile power, starting, and ignition systems is of secondary importance.
(source: Navy Electricity and Electronics Training Series)