From the US Naval Observatory website:
...The Earth's orbit is very close to being a perfect circle, but not quite. It is somewhat elliptical, which means that the distance between the Earth and the Sun varies over the course of the year. This effect is too weak to cause the seasons, but it might have some influence over their severity. The remainder of this page explains this possibilty.
Read all about it:
http://aa.usno.navy.mil/faq/docs/seasons_orbit.html