It is useful to represent the complete and specific 
funtionality that an actor(USER) is desirous of.
This functionality is shown by the USECASE model with the 
help of Diagrams, i.e. Sequence Diagram and Colloboration 
Diagram. It shows the various states of a work flow(Project 
or Program).

Diagram which represents the requiremnets(usecase) of the 
user(actor). its needed to represent the dynamic model at 
the analysis stage to represent the graphical view of the 
requiremnetal behaviour.

an usecase is a sequence of actions which can be done by a 
actor.

use case is a interaction between actor(USER) and System
(application), for single system there many use cases 
depends on Requirements.

use case represents diagrams. there are two types of use 
cases BUC-> business use case and SUC-> system use case
In BUC we represent pictures related to Domain and SUC we 
represents shapes like elipse,arrows etc. this is the very 
first stage of design where we find out actors and related 
activity. actor performs happy activity