PayPal Pro is a monthly fee based service +transaction fee's and Standard is just transaction fee's. Pro uses IPN which communicates with your website to verify the payment where as Standard does not.
Virtual terminal is also available, but has it's own monthly fee and transaction fee's applied.
As far as PayPal express goes, it also requires an API key which communicates with your website to validate payment. There is no monthly fee but there are transaction fee's. Either choose Pro or Express, not both.