A circle is actually a special kind of ellipse where the two focuses are equal. Every ellipse contains two points called a focus. The perimeter of the ellipse is the group of points, which are equally distant from these two focuses.
If a person were to select any point along the perimeter of an ellipse, measured its distant from the first focus, then measured its distance from the second focus, he would find that the sum of the two measurements would be the same no matter what point he picked along the ellipse.
In a circle, the two focuses have the same value, so if he were to pick a random point along a circle's perimeter to measure as above, not only would the sum be the same, but the measurements that he took for each focus would also be the same. This would be true all along the circle, no matter what point he chooses.
Ellipses have a major and a minor axis. The major axis is longer, while the minor axis is shorter. In a circle, each axis is of equal length.
With standard ellipses, the center point is off to the side of the focus points. In a circle, the center point is also the focus point.