WPS will mean "wireless provisioning service" which is related to hotspots and such. I'm presuming you have a feature that allows you to create one with that card. Here's a little more information about it. I'm not sure how it differs from cards that can be bridged to the wired network and used as access points. Why the button, I don't know. Maybe something in the instruction manual can tell you that.
I received a Ralink RT2860 draft n pci express wifi card for my pc about to days ago. I downloaded the latest drivers from the ralink website and installed them. Then when i tried to connect to my modem, it didn't even pick up the access point. My laptop with a intel wifi card picked up the accesspoint. I then did a test and created an ad-hoc network between my pc and laptop. The pc picked up and connected to the laptop but when i started moving away the signal dropped fast at the computer. At about 10meters the connection was lost.
The ralink card has 3 small antenas and i double checked that they are screwed in tight. Its also got a little button next to the antenas that says WPS. I have no idea what that is.