They provide a web browser-based user interface. To make this new UI concept work, Web Forms have lots of built-in intelligence. Controls on Web Forms run on the server but make their presence known on the client. These web interfaces can also have the intelligence to deal with different browsers, optimizing their output for each particular browser.
Supported browsers cover a broad range.
At the other end are simpler, less capable browsers on hardware such as wireless palmtop devices. Web Forms will render themselves appropriately on all of these. As with WinForms, Web Forms will be available to all languages. This brings complete and flexible web interface capability to a wide variety of languages. Server controls Visual Basic developers are familiar with the idea of controls.