Block diagram is a diagram of a system, in which the principal parts or functions are represented by blocks connected by lines, that show the relationships of the blocks. They are heavily used in the engineering world in hardware design, software design, and process flow diagrams.
The block diagram is typically used for a higher level, less detailed description aimed more at understanding the overall concepts and less at understanding the details of implementation. Contrast this with the schematic diagram and layout diagram used in the electrical engineering world, where the schematic diagram shows the details of each electrical component and the layout diagram shows the details of physical construction.
To make an analogy to the map making world, a block diagram is similar to a highway map of an entire nation. The major cities (functions) are listed but the minor county roads and city streets are not. When troubleshooting, this high level map is useful in narrowing down and isolating where a problem or fault is.
Block diagrams rely on the principle of the black box where the contents are hidden from view either to avoid being distracted by the details or because the details are not known. We know what goes in, we know what goes out, but we can't see how the box does its work.
In electrical engineering, a design will often begin as a very high level block diagram, becoming more and more detailed block diagrams as the design progresses, finally ending in block diagrams detailed enough that each individual block can be easily implemented (at which point the block diagram is also a schematic diagram). This is known as top down design. Geometric shapes are often used in the diagram to aid interpretation and clarify meaning of the process or model. The geometric shapes are connected by lines to indicate association and direction/order of traversal. Each engineering discipline has their own meaning for each shape.