Instead of using a single equation to generate a curve, two parametric equations are used.
Here the letter t denotes the parameter, and the x- and y- coordinates are calculated from the formulas:

x = A cos(Bt) and y = C sin(Dt)

In other words, the coordinates of each point on the graph are of the form: (A cos(Bt), C sin(Dt)) for different values of the parameter t.

You can vary the values of A, B, C and D using the buttons at the bottom.
By varying A and C you can create a series of circles and ellipses.
Varying B and D complicates the situation by drawing the strikingly beautiful Lissajou Figures.
Play around - enjoy yourself!