A database (and therefore an ASP data-driven site) is only as good as the data within it. For example, if you collect information from your users (like an eCommerce site, let's say), and your users enter incorrect or malformatted data (such as an incorrect email address or an invalid zip code), the data is as good as garbage.
For that reason, it is important that you verify that the data entered from your users is, at minimum, in the correct format. This process is known as form validation and can occur in two places: through client-side scripting code on the user's browser (known as client-side form validation) and on an ASP page before submitting the data to the database (known as server-side form validation). The articles linked to here provide information on both form validation techniques and are all definitely worth a read!
Client-Side Form Validation
Server-Side Form Validation
ASP.NET Form Validation Articles (for more information on ASP.NET visit the
ASP.NET Article Index!)