Create a parabola from Lines and a Right Angle. Via...

Create a parabola from Lines and a Right Angle. Via WonderHowTo.

EDIT

As some readers have pointed out, there is a typo and the curve is actually an hyperbola.

Well, the easiest way to notice the difference (at first glance, I mean) is through the parametric equations:

Parabola: X = t² and Y = t 

Hyperbola: X = t and Y = 1/t 

and realize that the asymptotic behavior of the hyperbola (at t→ 0 and t→ ∞) on the graph is not given in a “parabola” (the first five drawings).

In any case, to avoid problems, the author of the article should have been more generic (and careful) and have simply written “conic sections" or "quadratic functions”.

Thanks anyway for the remark.