You pay $79/year (or $39 if you have a .edu email address) and you get free two day shipping on anything that is sold by or fulfilled by amazon (corresponds to pretty much any item you get "super saver" shipping on now) and you can choose one-day shipping for only $3.99. You also can stream movies and tv shows.
Amazon.com: FREE Two-Day Shipping, Instant Videos, and Kindle Books with Amazon Prime