The two foci of an ellipse are the two points in the center of the ellipse. The sum of the distances from the two focii any point has on the ellipse is constant. In a planetary orbit, these are the two points which the orbit's ellipse is centered around. For example, in Earth's orbit, the Sun is one of the focuses.

The eccentricity of an ellipse is a measure of how squashed the ellipse is. It is half the distance between the two foci divided by the length of the semi-major axis picture below. In a planetary orbit, if the eccentricity is 0-1, the orbit is a squashed ellipse, 1 is a parabolic orbit, and >1 is a hyperbolic orbit.

The perihelion is the distance from one foci to the closest vertex of the semi-major axis.

The aphelion is the distance from one foci to the farther vertex of the semi-major axis.

In planetary orbits, the perihelion is the closest point to the star/sun in the planet's orbit, and the farthest point is the aphelion.