Paypal API vs. IPN for payment processing

I'm setting up my first Pro Site, and noticed that I need a Paypal IPN account and I can only have one of those???

First, what is the difference between Paypal API vs. IPN?

Is there any other plugins that require the IPN account besides the Pro Sites? If yes, will I be able to use the same one I'll be using for Pro Site?

The API, I have one for Directory I set up. Are we limited on those?

Anything else I should know?

Thanks,
Tom