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.