ASP.NET validation controls define an important role in validating the user input data. Whenever the user gives input, it must always be validated before sending it across the various layers of an application. There are two types of validation in ASP.NET:

Client-Side Validation
Server-Side Validation
Client-Side Validation: When validation is done on the client browser, it is known as Client-Side Validation. You can use JavaScript to do the Client-Side Validation.

Server-Side Validation: When validation occurs on the server, then it is known as Server-Side Validation. Server-Side Validation is a secure form of validation. The main advantage of Server-Side Validation is if the user bypasses the Client-Side Validation, the problem can be caught on the server-side.

The following are the Validation Controls in ASP.NET:

RequiredFieldValidator Control
CompareValidator Control
RangeValidator Control
RegularExpressionValidator Control
CustomFieldValidator Control
ValidationSummary