A Component diagram illustrates the pieces of software, embedded controllers and such that make up a system, and their organization and dependencies.
A Component diagram has a higher level of abstraction than a Class diagram; usually a component is implemented by one or more Classes (or Objects) at runtime. They are building blocks, built up so that eventually a component can encompass a large portion of a system.
Example Diagram Example Component Diagram
Select Component diagram elements and connectors from the Component pages of the Toolbox.