Activity diagram is a behavior diagram, which shows a scenario using a flow of actions. It models a sequence of actions, decisions based on conditions, concurrent branches and various cycles.
Sequence diagram models communication among objects in sequential order. It shows object, classes and actors involved in the scenario and exact order of messages. It consists of vertical lines which belong to lifelines and horizontal lines of messages.
Use case diagram consists of use cases, actors and relationships among them. It presents a user interaction with the system and defines the specifications of use cases.
Class Diagram is a static structure diagram which describes the structure of a system by showing its classes with their attributes and operations, and the relationships among objects.