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.