What is the difference between PayPal payment options: Pro, Plus, Checkout, Advanced Integration?

I've been reviewing the aMember PayPal payment plugin options (paypal, paypal-express, paypal-plus, paypal-pro), and trying to research the different PayPal seller options. Is there somewhere that I can get a concise description of exactly what these different payment options are?

I currently use PayPal with aMember, and it works fine. I would like to integrate payments more tightly with my website look and feel, and possibly pay a monthly fee in exchange for lower per-transaction fees. What should I be researching?