There are three different Milankovitch cycles which describe the orbital variations in the rotation of the Earth; eccentricity, precession, and axial tilt. Together, these cycles determine the amount of solar radiation each part of the world receives.

Eccentricity is the shape of the Earth's orbit around the sun. This ranges from nearly circular to elliptical and is caused by the gravitational pull of the other planets. A complete cycle of circular to elliptical and back to circular lasts 100,000 years. When the orbit is elliptical, winters are colder and summers are warmer.
Because the earth is not a perfect sphere, it wobbles on its axis, similar to a spinning top. This wobble is called precession and the Earth completes this cycle in 26,000 years. This cycle is caused by the gravitational pull of the moon. Seasons are impacted by this; summers can become warmer.
Axial Tilt is the angle at which the Earth's axis is pointed. This varies between approximately 21 degrees and 24 degrees over 41,000 years. A greater degree of tilt results in the Earth having a higher average temperature.