Hi
Dotnetnuke is pure .Net, specifically ASP.NET. Dotnetnuke does exactly what you could do yourself with .Net only better. Imagine you would like to code a login or an e-commerce site for a client. YOU would be responsible for all that complicated stuff like session management, control state & cryptology, not to mention adhering to design patterns and best practises and accounting for any possible known bugs or exploits etc etc.
The benefit of using DNN IMHO is that you would be in a much stronger position if you rather familiarize yourself with DNN instead of fighting tech so to speak. In other words, using already implemented tested code using biggest ASP.NET community in OSS, you can leverage tech using your programming debugging expertize and deliver cutting edge tech without all the hassles and complexity. To code something like DNN yourself without all the free extensions from the community would be near impossible. Close relationship with Microsoft also allows tight integration with modern day APIs and web, tablet and mobile devices.
So yes to answer your questions you can create customized forms and load them into dotnetnuke by creating a module with a manifest file to install into DNN and then you could take advantage of session management and a bunch of other APIs as well, too many to mention. There are training videos, tutorials, webcasts, forums, manuals and blogs and websites to learn. Then you could use the provider model or install your own e-commerce, database apps.
Hope this helps. let me know if you have any other questions, for tech or business case....