client means our customer.end user means they did front end 
work.

client is customer to us




client have lot of customers himself







end users are nothing but who are using the application for 
their business process

client is customer to us

client have lot of customers himself
end users are nothing but who are using the application for 
their business process

Client - who give the product to us.
Customer - Client sell this product to some people. They 
will become customers.
End User - Those who use the product are end users.

Take an example for easy understanding.
Hilton hotel wants a s/w for their business operations. 
Here Client is Hilton Hotel
     End User is Who will book rooms online in Hilton Hotel 
are ,nothing but WE.
     Customer ,here may be Agents

Client : Our customers.

Customer : client's customers or our customers.

End User:Who will actually use the application.