In .NET we do validations to restrict wrong entries and making valid values to be entered through the forms.

Validation on a form or UI page can be done on two sides

1 Server Side Validation

2 Client Side Validation

Server Side validation we provide when we want to check the details after making request to the server

For providing server side validation we will use ASP validators

Ex :  <asp:TextBox id="txtName" 
             runat="server"></asp:TextBox>

            <asp:RequiredFieldValidator 
             id="RequiredFieldValidator1" runat="server" 
             ErrorMessage="Required!" 
             ControlToValidate="txtName">
            </asp:RequiredFieldValidator>

Client side validation we provide when we wan to check the details as they are entered instantly into the form

For providing client side validation we use Javascript validators or Jquery validators

Ex :  <script> 
       function checkname()
       {
  var name=document.getElementByID("txtName").value;
  if(name=="")
{
alert("Name cannot be left blank");
}
      }
     </script>