Ira,
Not a dummy question at all.
Basically there are two types of ring flashes. The larger
Alien Bees type of ring flash is intended for portrait use. A small ring flash, like the Sigma, is for closeup and macro work.
You can certainly use the Sigma for portrait-type of work, but realize that the index number of the flash is quite small. You will have to get more in the face of the subject literally and that may be uncomfortable.
Also realize that the shots below were taken with 35mm and a 60mm macro lenses respectively. A 90mm may force you to stand far enough away to lose the effect of the light. But there is nothing stopping you from using another lens with the flash.
My kids, being
my kids, have suffered my grabbing a few snapshots with the Sigma and I really like the effect. My wife framed the one of my son below. Not for everyone's taste, but different.
M