client is an device  which is resource of OOP( object oriented programming ) and it is an also a program which provides services by server . 
         server is a computer channel or network based service by which we make access files , communication , data , etc. to users and programers .  
              i.e. in short we say that client is requesting user and server is supplying user .
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a computer or computer program which manages access to a centralized resource or service in a network is called a server

 a desktop computer or workstation that is capable of obtaining information and applications from a server is known as client