Fundamentally an ASP.NET page is just the same as an HTML page.
An HTML page has the extension .htm. If a browser requests an HTML page from the server, the server sends the page to the browser without any modifications.
An ASP.NET page has the extension .aspx. If a browser requests an ASP.NET page, the server processes any executable code in the page, before the result is sent back to the browser.
HTMl pages are uses for static view of content...While ASP.Net pages are using for dynamic geneate pages
You can't use an html page for connecting to a database because you can't write any server-side code in it.Like those guys have said above html pages are static.To change their content you have to edit the code(html).On the other hand, asp.net pages can also be static but if they derive their content from a data source like a database they work like magic.Used to build dynamic websites that interact with the user and service user requests.