Client side validation When you place validation code on the client side, validation does not require a postback operation and provides fast responses to the user.
This client-side validation should never be used as the only validation technique to validate data on a Web page.
Server Side Validation When the validation code is placed the server side, the process of validation might be slow because a form might involve multiple roundtrips to the Web Server before all the data is validated.
A good example will be a Text Box which is used for taking the username as the input.
Since the user must enter the username in the Text Box in order to access the website.
You can always turn the validation on the client side off using the enable client side property to false.