One of the coolest things about ASP.NET is using Web Controls. These are replacements for using traditional HTML controls such as a button, textbox etc.. Have you ever had to needed to have a field needs to be filled before its submitted. Using Web Controls has a control called <asp:RequiredFieldValidator> that if put right after the server control will ensure the field has a value to fill text box
One of the major benefits about using ASP.NET web controls is the validation route. Depending on what browser requests the page, the ASP.NET engine will render client-side javascript code automatically to validate the form on the client. If the browser isn't capable to handle the client side validation routine, the form is submitted to server causing a round trip.
This simple example just shows the concept of requiring one field to be filled in. Some of the advanced things that also can be done is using Regular Expressions to further validate the data in the textbox, checkbox or radio button(s).