There's three of them depending on time control:
Online Blitz (3+2 through 10+0)
Online Quick (10+1 through 29+0)
Online Regular (30 or above)
Online Regular is fairly new so that's perhaps why you read there were only two. But nowadays there's three.
USCF has 3 online ratings, online regular (Classical), online quick (rapid), and online blitz (Blitz). You can find your online ratings in the same place you find your otb ratings. Look up your name on uscf, and find your profile.
Hey, thanks guys! 
Hi, I seem to remember reading somewhere that there are two(?) categories for Online USCF Ratings...is this true? And if so, then what are they? (Rapid, Blitz, etc.)
I've tried asking multiple Administrators but no reply has been forthcoming, so I'm hoping one of my felluw US Chess people can help answer this...