First read Webster 1913's epicycloid writeup. The cardioid is the epicycloid for two circles of equal radius a. It is heart-shaped in appearance and has equation

(x2 + y2 - 2ax)2 = 4a2(x2 + y2).

Like all the epicycloids it is interesting from the point of view of gears. It is a special case of the limaçon of Pascal.