A block diagram uses a drawn box to represent complicated sections in a device or process, and the connections between the sections.

For example:

           ________
           |Upvote|----------+      
________   --------        __|_____
|Select|    |              |Press |
| Vote |----+              | Vote |--Done
|Option|    |              |Button| 
--------  __________       --|-----
          |Downvote|---------+
          ----------

This can be used for items such as radio or radar systems to depict sections of the electronics, such as receiver circuitry, without showing a complete schematic.