1. A circle is a special ellipse with a centre and equal height and Width called diameter. ellipse(x, y, diameter, diameter)
  2. Arc is part of a circle (or ellipse). arc(centerX, centerY, width, height, startAngle, stopAngle) 
size(500, 375);
smooth();
background(255, 204, 0);
ellipse(250, 188, 200, 200); 
strokeWeight(5);
arc(250, 188, 200, 200, PI/4, PI/2);