Sequence Diagram

** Sequence diagram models the sequential logic, ordering of messages with respect to time.
** Sequence diagram mainly represent interaction between different objects.It is time ordered means exact interactions between   objects is represented step by step.
** Sequence diagram describes the behavior of several objects in a single use case.
** sequence diagram is lifeline of the the object and it is dynamic modeling.
** Sequence diagrams describe interactions among classes in terms of an exchange of messages over time. Sequence diagrams show a detailed flow for a specific use case or even just part of a specific use case. They are almost self explanatory; they show the calls between the different objects in their sequence and can show, at a detailed level, different calls to different objects.

Activity Diagram

** Activity Diagram mainly represent process flows captured in system.
** Activity diagrams are good at showing the general sequence of actions for several objects and use cases.
** Activity diagram is not dynamic diagram and it is used to see the workflow of the software.
** Activity diagram high-level business processes, including data flow, or to model the logic of complex logic within a system.
** Activity diagrams illustrate the dynamic nature of a system by modeling the flow of control from activity to activity. An activity represents an operation on some class in the system that results in a change in the state of the system. Typically, activity diagrams are used to model workflow or business processes and internal operation.