The change in seasons is caused by the varying angle of the Earth's tilt from one part of the year to another. A common misconception is that the change in seasons is due to the Earth's slightly elliptical orbit bringing the planet closer to the sun in summer, but, as About.com notes, seasons are different in the northern hemisphere and the southern, which are the same distance from the sun.
During summer in either the northern or southern hemisphere, that hemisphere is inclined toward the sun relative to the planet's orbital plane. About.com describes the effect of this tilt as putting the peak of the sun higher in the sky, which causes longer days and gives the sun more time each day to heat the surface in that hemisphere. The opposite process is in effect during winter, when the Sun spends less time in the sky each day and therefore heats the surface less.
The seasons are local phenomena, with winter in the southern hemisphere corresponding to summer in the northern. The Earth does, in fact, have a slightly elliptical orbit that brings it closer to the sun during the northern hemisphere's winter months, causing somewhat milder seasons in the northern hemisphere.