Form validation occurs at two levels in an ASP.NET web form: client-side and server-side.
The value of client-side validation is: 1) the user immediately sees whether the data they entered is valid instead waiting for the browser to submit the form to the server, waiting for the server to perform validation and resend the form to the browser, and 2) simple field validation can be performed at this level. The disadvantages are: 1) validation involving multiple fields is more challenging or impossible to code for client-side execution.
Server-side validation is performed as indicated on the server. When the user clicks on the Submit button all validation controls are executed. If validation controls are successfully the browser submits the form data to the server. The ASP.NET code on the server examines the form data and determines whether it is valid. If the form data is invalid the ASP.NET code usually inserts an error message at the top of the web form telling the user about the problem.