Component and deployment diagrams are different UML diagrams. They both show components - but at different levels.

Component diagram shows components and dependencies between them, interfaces they provide (implement) or require, classes that realize component. It has no deployment nodes - as deployment diagram does.

Deployment diagram shows deployment architecture of system - how components and artifacts are deployed to nodes - execution environments and devices.

Though we can show dependencies between components both on component diagrams as well as on deployment diagrams, but we are not showing implementation of components on deployment diagrams as we do on component diagrams.

Note, that in UML 1.x deployment diagrams components were directly deployed to nodes. In UML 2.x artifacts are deployed to nodes, and artifacts could manifest components. So, on deployment diagrams components are now deployed to nodes indirectly through artifacts, and generally speaking in UML 2 deployment diagram could show only nodes and artifacts, and no components.

Simply put, a Component diagram shows you how different elements of your system have been grouped together (into assemblies / dlls etc) - and the link between these components. A Deployment diagram takes you one step further and describes on which hardware elements do these components reside.

So for example, if "Utility.dll" is a component and say it is deployed on the Client Machine (hardware). Then, the Component Diagram of this system will show Utility and its link with other components in the system (say.. Customer / SQL Packages). Whereas, the Deployment Diagram will show the hardware configuration - DB Server / Web Server / Client Machine .. and Utility component will be placed into the Client Machine Node.