I am looking at APR details and want to understand how it works.

I see a credit card that says:

• Introductory APR - 0% for 15 billing cycles
• Standard APR - 14.74% - 24.74% variable
• Balance Transfer Fee 3% of each transaction (minimum $10) ( I think this is to pay another credit card)

I want to understand what each one means. Plus also I want to understand that if I pay BEFORE the due date, I will not be charged any interest.