A block diagram is a type of a system in which the main components or works are represented by cubes connected by lines which reveal the relationships of the cubes.
Block diagrams rely upon the principle of the black box in which the contents are hidden from view either to avoid being distracted by the facts because the details aren't known. We understand what happens, we all know what goes out, but we can not see the way the box does its work.
A good illustration of that is that the function block diagram, among five programming languages found in section 3 of this IEC 61131 (view IEC 61131-3) standard that's highly formalized (see formal system), with strict rules to how diagrams are to be built. Directed lines have been used to link input variables to block input signal, and block outputs to output variables and inputs from different cubes.
In biology there is an increasing use of technology fundamentals, techniques of investigation and methods of diagramming. There is a similarity between the block structure and what's called Systems Biology Graphical Notation. As it is there is use made in systems biology of the block structure technique exploited by controller technology where the latter itself is an application of management theory.
In process control, block diagrams are a visual vocabulary for describing actions in a complex system in which blocks are black boxes which represent mathematical or logical operations that happen in order from left to right and top to bottom, although not the physical entities, like processors or relays, that perform those operations. It's likely to create such block diagrams and implement their performance with technical programmable logic controller (PLC) programming languages.
In electrical engineering, a style will often begin as a quite high level block diagram, becoming increasingly more detailed block diagrams because the design progresses, eventually ending in block diagrams detailed enough that every individual block can be readily executed (at that stage the block diagram will be also a schematic diagram). This is referred to as top down design.  Geometric shapes are frequently utilised from the diagram to help interpretation and clarify meaning of the procedure or version. Each engineering field has their own significance for every shape. Block diagrams are employed in every discipline of technology. They are also a valuable source of theory building and educationally valuable in non-engineering areas.
Block diagrams are typically used for higher degree, less detailed descriptions which are meant to describe general theories without difficulty for the details of implementation. Compare this with the design diagrams and layout diagrams used in electrical technology, which show the implementation information of electrical parts and physical construction.
As an example, a block diagram of a wireless is not expected to demonstrate each and every link and dial up and switch, but the design diagram is. The design of a radio doesn't show the diameter of every connection from the circuit board, but the layout diagram does.
To create an analogy to the map manufacturing planet, a block diagram is comparable to a highway map of an whole nation. The major towns (functions) are listed but the small county roads and city roads are not. When troubleshooting, this high degree map is useful in narrowing down and isolating where a problem or malfunction is.