ASP.NET Validation Controls are powerful server controls that are used for validating user input. These controls provides both server side and client side validation.
The client side validation features can be used to give your users an improved UI experience. By using the client side features, your users will not need to wait for a full page postback to occur only to discover that they have failed to fill out your form completely. Instead, the client side features can be using to notify the user immediately (before the page is submitted) about any missing and/or invalid data.
One of the shortcomings of the validation controls under the 1.1 Framework was the inability to validate only portions of your form. Whenever the validators were triggered, all validators on the page were checked even if they might not be related to the particular button being clicked. With the 2.0 Framework, a new ValidationGroup property was introduced. By setting the ValidationGroup property on a button, only those validation controls that were also set to that same ValidationGroup will be checked.
The validation can be checked on the server side with the help of the IsValid property of the validation control. Additionally, you can use the IsValid property of the Page to check the validation result of all validation controls on the page. Before testing the IsValid property of the validation control or the page, it is important that the validation check has already been performed through the use of the Validate method. You can call Validate on individual controls and also on the page itself.
Here is a list of the validation controls