I checked the wiki for the Net and Gross product and all it wants to show me is the gift certificate tutorial.

I really don't understand the difference between net/gross. I put a value in one and the other creates a calculation of lesser/greater value. I have a few questions;

What is causing the calculation?
How can I eliminate the varation and have 1 price box?
Is there a place I can put a "cost" price so that I know what the item was paid for without the customer seeing?

Thank you for answering noob questions.