This GGB applet shows how (and more importantly, WHY) a parabola is defined as a locus of points in the plane that are equidistant from a fixed point (focus) and given line (directrix). Students can drag point D along the directrix and watch the trace of P (point on the parabola formed.)
You can also move the directrix and focus around (if you'd like), and then drag point D along the relocated directrix to form a new parabola.
