Interactive web pages add value to a site, here are some examples:

Search Engines
A search engine allows the customer to find the product they need amongst possibly many hundreds of products, thus saving them time and allowing them to browse for other products without hassle.

Validated Forms
Validated forms provide an automatic way of getting data back to a database or office in the correct format with all the necessary information, which saves your staff time. Forms can be processed automatically (this can be over a secure link) or sent by email (unsecure).

ASP Pages
ASP pages are the same as html pages, but parts of them are automatically filled in by the server before being sent to the client. This method allows a large number of users to access interactive content and most importantly allows corporate databases to be directly linked to web pages via software running on the server. This is the main diasadvantage of ASP, if your are a small company without your own Internet server you cannot use ASP directly.

Guest Books
Guest books are less important than they used to be especially on large sites, because as the number of visitors increase so does the guestbook size - and who wants to look through a guest book with hundreds or thousands of new names a day? - But still, guest books are can be good for small sites.