The basic shape primitive functions are triangle(), rect(), quad(), and ellipse(). Squares are made with rect() and circles are made with ellise(). Each of these functions requires a number of parameters which determines their position and size.
Original Processing.org Example: ShapePrimitives
// All Examples Written by Casey Reas and Ben Fry // unless otherwise stated. size(200, 200); smooth(); background(0); noStroke(); fill(226); triangle(10, 10, 10, 200, 45, 200); rect(45, 45, 35, 35); quad(105, 10, 120, 10, 120, 200, 80, 200); ellipse(140, 80, 40, 40); triangle(160, 10, 195, 200, 160, 200);