Not that long. The first tutorial post I read explained it quite well. Unfortunately it's been so long I can't remember who wrote the tutorial or I'd recommend it. There are other great beginner tutorials on steemit I am sure so any of them should explain the difference.
This is something different, STEEM is the currency used in steemit, while steem power is your power level on the steemit platform, affecting how much you can curate a post.
I understood it instantly because everything is well explained on wallet page.