What does WS-Federation really do (in depth and by a simple & understandable example)?

I have read some text about WS-Fedaration but i can not understand it. I have some questions :

1. What would happen if there were no WS-Federation?
2. How does it help to Single Sign-on?
3. What is the difference between WS-Trust and WS-Federation?

I just want answers in a very very simple and understandable samples in the real world! I have read a lot but i can not understand it deeply

Thanks