An ellipse: the distance A from one focus to a point on the ellipse plus the distance B from the point to the other focus stays constant as you go around the ellipse.
You can make an ellipse on paper with a string and a pencil.
The length a in the picture is called the semimajor axis of the ellipse.
Ellipses can have different shapes, usually characterized by their "eccentricity." A circle is a special case of an ellipse. It has zero eccentricity.
Here are two orbits that are ellipses with the sun at one focus.
The orbits of most of the planets are nearly circles. But other objects, especially comets, have orbits with high eccentricities.