Server-side code runs on the server. Client-side code runs
in the clients? browser.

Server side validation can be done on server side controls
using server controls.
But Client side validation can be done at browser.

The control validation can be done at client browser instead
of server side validation.Because its additional overhead to
the server.

For example,Mandatatory fields checking.It can be done at
client browser.No need at server side.

But the login and password must be validate at server side.

The client side validation can be done using the Javascript
or vb script.

Control or object created by browser is called client side 
control

control or object created by server[web server] is called 
server side
control

Server Side Code 
Server side is the code that resides at web server. 
For every client request code is executed at server side 
and result is send to the client in simple HTML format. 
Performance is lower than client side code due to server 
round trips. 
Client cannot see the business logic though it is stored on 
server. 

Client Side Code 
Client side code is reside at client's browser itself. It 
is executed at client side only. User can easily see the 
code by View - > Source option. 
It is generally used in validation form like text field is 
empty or not, email address validation etc. It is faster 
than server side code.

server side code is responsible to execute and provide the 
executed code to the browser at the client side. the 
executed code may be either in XML or Plain HTML. the 
executed code only have the values or the results that are 
executed on the server. The clients browser executes the 
HTML code and displays the result. 

where as the client side code executes at client side and 
displays the result in its browser. it the client side core 
consist of certain functions that are to be executed on 
server then it places request to the server and the server 
responses as the result in form of HTML.

server side code executes on server side while client side 
code executes on client side

Server side code executes on the server.Client side Code
Executes on the client Browser.