The reason you may be struggling to find an answer is the terminology you're using. The term
is generally reserved for Windows applications. When developing web applications use the term
. Whilst a web page often contains a form, it is a component of the page. When talking about loading content from the server, you're loading a page, not a form.
There are many panel components available which allow you to asynchronously load content from other pages and display them in your current page. One such component is the UpdatePanel. See the following link for more details:
UpdatePanel Control Overview
If you want to know more about what these components are doing behind the scenes then you need to study writing AJAX code.
Here is a good starting point:
Another way might be to use an iFrame. An iFrame is essentially a browser window inside a browser window and my be suitable for your purposes although are a bit old hat nowadays.