Client : Usully one who make requests.
         In very simple language....if u r sitting in   
front    of a computer connected to the internet...and u 
type  in MAIL.YAHOO.COM in ur browser...then thats the 
functionality of a client....in short ur computer is ur 
client sending requests.

Server : Its the one which proceeses the request sent by 
the client ...does some activity...and responds to the 
requests.All web based applns have a server.when u type 
YAHOO.COM ur request is sent to the YAHOO ASIA SERVER 
LOCATED IN SINGAPORE.it will process it and then send u the 
response that u see opening up the yahoo home page.
In case if the server is busy...coz...each server has a 
particular load capacity...u get a apage cant be displayed 
or server not found message.

Usully all the client's requests are sent to the server via 
TCP/IP Protocols.

Hai friendz I am a rajendra

Client-server it is network architecture
Client-server is a two tier architecture as it name 
indicates 
Client and server are two important important components in 
client server architecture.

Client is nothing but an application program running in a 
Computer which can access data or files from server.

Client application program sends requests to the server.

Client application program provides a front-end to access
Data from server.

Server processes the client requests and send information 
to the client.

TCP/IP or HTTP protocol is used for communication between 
client and server.

In a Nutshell client is a system which has a tool i.e. 
application program to access data from server.

Server processes client requests and sends requested 
information to client     

but i want say one thing without diagramatic explanation
it is easy expaln clent-server architecture.

Finally

        applications architectures are three types
           1.Two tier architechture(clent server )
           2.Three tier architechture.
           3.N-Tier architechture
nat test engineer should have a fair knowledge about above 
architechtures bcoz any applications/products will fall 
any one of the above stated architechture.

still you have any doubt mail me

at :
  rajendra_penumalli@yahoo.com

ring me at :
              9885162742
Hyderabad
A.P