Skin vs Theme

hi all

I am thinking of my niche for my first blog and read about thesis 2 theme. From what I can see, it looks clean and has a lot of user friendly features. After surfing some more and listening to podcasts I heard of marketers delight which is a skin for a theme.

As a noobie, I am wondering what exactly is the difference between the theme and the skin? Which sets my blog’s color scheme? What does a skin give me that is missing in the Theme?