Ok, this might sound as a silly question, but I am a newbie without experience, and I am curious how stuff works, I am not for the "just works".
The default website generated by VWD2010 is documented somewhere? I searched in MSDN but no luck.
If i understand how it works, it would be better, to understand how login is implemented, the master page, and so on.
Sounds like you're looking for tutorials. I suggest Microsoft's free tutorials.
When you create a new website, the IDE creates that website using a template. The template has code for the website. For example, creating a .css file and linking that. Look through the files that are generated and look at the code that is there.
I suggest you start here at the Get Started page for ASP.NET.