circle at top of screen

Hey guys,

I just got my iphone last night and I have no idea how to use it. I have a small circle at the top of the screen where the "3G" was before. I have 3 out of 5 bars and am in an area where I usually have good service with my carrier (Sprint). Why is the circle there? Siri told me that she can not respond at the current time. Also, why do so many apps seem to only work when I have wifi? I pay for wireless service so that I can connect to services outside of a WIFI network.

Thanks for any help guys. And, any additional tips regarding iphone would be awesome. I am so naive to this new device!