Q2. differences

component diagram - possess the physical analogs of class
- is more of a bird's eye view of the system and the component of the system

class diagram - show the classes of the system, their inter-relationships, and the operations and attributes of the classes
- gives an overview of a system by showing its classes and the relationships among them
- are static and they display what interacts but not what happens when they do interact.

example of component diagram:



source: http://www.sparxsystems.com.au/resources/uml2_tutorial/uml2_componentdiagram.html

The diagram above demonstrates some components and their inter-relationships. Assembly connectors "link" the provided interfaces supplied by "Product" and "Customer" to the required interfaces specified by "Order". A dependency relationship maps a customer's associated account details to the required interface; "Payment", indicated by "Order".