A structure diagram is a conceptual modeling tool used to document the different structures that make up a system such as a database or an application. It shows the hierarchy or structure of the different components or modules of the system and shows how they connect and interact with each other. It is a tool used to guide developers to ensure that all parts of the system work as intended in relation to all the other parts.
Techopedia explains Structure Diagram
A structure diagram is a development tool used in modeling the different parts of a system, from the overview on how the individual parts interact to create the whole, to modeling the details of the smallest parts themselves such as the different objects and classes being used in programming the system.
A structure diagram visualizes how a system works from the initial input, to processing and, finally, to the desired output. It is especially useful in determining all of the interfaces involved between the different parts and helps developers agree on how each part should be connected based on the models being shown on the structure diagram.
There are various kinds of structure diagrams, including the following:
Data structure diagram - A conceptual data model used to document the entities in a database, their relationships, as well as the constraints associated with them
Composite structure diagram - A type of static structure diagram that shows the internal structure of a class and its collaborations with other classes and objects