Power up 100% or Default 50% 50%. What does it mean.

I am here on steemit trying to find more information on how the payout works on steemit.
I have a few questions?
What is the difference between power up 100% or default 50/50?
What post is it better to power up or default?
Is there a reference page to visit to get a better understanding of how all areas in steemit works?

So please can someone clearly explain to me how the payout works and whats the best way to make and save up your money before taking a pay out.
Thank you.