How to explain Katana and OWIN in simple words and uses?

I have read many articles about the OWIN and Katana projects, but I could not get the whole picture of it.

For a normal web developer who uses ASP.NET:

1. What exactly is OWIN and what problems does it solve (in simple words). What is its relation to IIS?
2. Does OWIN replace IIS? if not, in what situations does OWIN best fit?
3. How could OWIN help me in my daily work projects?
4. How could OWIN help me in a self-improvement projects?