API architecture diagrams are visual representations of the structure and interactions of an API's components. They help developers plan interface structures, users understand intended use, and QA engineers test and debug APIs. Common types include sequence diagrams, service architecture diagrams, and internal API architecture diagrams. Tools for creating these diagrams include Unified Modeling Language (UML), Mermaid, and Lucidchart. Best practices involve using the right type of diagram, considering accessibility, keeping it simple, and avoiding jargon.