Difference between Paypal IPN, paypal and express?

I'm currently setting up my store and testing, I have as payment options Paypal IPN, Paypal and paypal express. What is the difference between all of these and should I really only have one switched on? During testing they all lead to paypal so are they essentially all the same?

I'm not familiar with the module you're referring to as just "paypal", but there is a difference between IPN and Express Checkout.

Express Checkout offers your visitors an opportunity to checkout through PayPal without creating an account at your store. PayPal IPN requires a store account, and directs them to PayPal to complete payment.

I believe that Express Checkout also offers an option to pay with PayPal to account holders during checkout, so if you have Express Checkout I believe you can disable IPN.

Hope this helps,
Joe