Ignore assassin, he hasn't grasped the magic of google.
I've tried to explain it to him -_-
e = (r_aphelium - r_perihelium)/(r_aphelium + r_perihelium)
r_aphelium: distance to the sun at aphelium
r_perihelium: distance to the sun at perihelium
aphelium: point of the orbit furthest away from the sun
perihelium: point of the orbit closest to the sun
The sun is located at one of the focal points of the ellipse. If e = 0, the ellipse is a circle.