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Server-Side and Client-Side Validation
Validation controls perform input checking in server code. When the user submits a form to the server, the validation controls are invoked to review the user's input, control by control. If an error has occurred in any of the input controls, the page itself is set to an invalid state so you can test for validity before your code runs.
If the user is working with a browser that supports DHTML, the validation controls can also perform validation using client script. This can substantially improve response time in the page; errors are detected immediately and error messages are displayed as soon as the user leaves the control containing the error. If client-side validation is available, you have greater control over the layout of error messages and can display an error summary in a message box. For more details, see Client-Side Validation for ASP.NET Server Controls.
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