.Net (or the WinForms parts, anyway) sits on top of Win32. Or, put another way, Win32 was used to build .Net forms components. So in that sense you can think of .Net as a set of pre-built win32 widgets. You can also think of .Net as the logical successor to MFC.
.Net also has the ability to call into the Win32 API directly when necessary, gives you garbage collection, a very nice class library in the BCL, and a lot of nice language features over C/C++ in C# and VB.Net.
What you lose to get all these things is a certain amount of independence. .Net is an add-on framework that is not shipped with all versions of windows by default, and therefore you have extra dependencies to worry about at deployment. You also have performance considerations to think about when using any high-level garbage collected language, where seemingly simple code might be doing a lot more than you expect behind the scenes. This is especially true for some of the winforms components.
