Answer 1

http://www.wpf-tutorial.com/about-wpf/what-is-wpf/
It is a UI framework.
So it is a way to described the visible part of your application.
For programming in c# you can choose between WPF and winforms.
The advantage of WPF is that the form is scaleable so it looks good on different device for example phone, table, laptop.
It stimulates you to separate the UI-layer and Data-layer. Which is good for maintainability, because of this WPF does have a steep learning curve.
The other method to make forms for c# is Winforms.
I hope sololearn will make a course for WPF in the future.