If a beam of light is linearly, circularly or elliptically polarized, as the electric field vector propagates it will trace out a line, a circle, or an ellipse about its axis of motion.  By superimposing two light beams of comparable frequency, such as the fundamental and second harmonic that are obtained through Second Harmonic Generation of a single beam, new polarization states are created that follow Lissajous-curve like behavior (such as the trefoil pictured here).