For example you can model life-cycles of an object having different states. A statechart diagram consist of states and transitions between them. Initial state shows the start of life cycle and final state describes the end. History state presents the previous substate in embedded states. All other states are normal states between initial and end states.
State transitions describe events that cause an object to go from state to another.
Component diagrams A component diagram is used in modeling physical aspects of the system. With component diagrams you can visualize the implementation of component-based system. A component is a physical part of the system that can realize a set of interfaces. A component is easy to replace with another component as long as it implements properly its interfaces. A component can be an executable, a library, a source or data file or any other physical part of the system.