usually is just a 2-dimensional disk. the most general way to represent it in the Euclidean plane is
, where the inequality is strict if it is an open disk.
for convenience the center (h,k) is often taken to be (0,0) and the radius taken to be 1.
i am guessing that the other set is or its equivalent complex form,
, the unit circle, which is the boundary of the unit disk.