Winter covers the months of June - August in the southern hemisphere, and December to February in the northern hemisphere.
Winter officially commences after the winter solstice - the shortest day of the year. It is characterised by shorter days and colder nights due to the earth being tilted at 23 degrees away from its eliptical path around the sun. This result in the atmosphere in the particular hemisphere receiving a reduced amount of energy per unit of the earth's surface, as the sun is falling on it more obliquely than in summer.
In the northern hemisphere during winter, the earth is actually closer to the sun than in winter (due to the earth's slightly eliptical path around the sun), although it is the angle at which the sun hits and diffuses across the atmosphere that is more important in determining the the seasons in a given region.
The reduced amount of daylight is a trigger for deciduous trees to drop their leaves to conserve energy, just as many mammals go into hybernation. Winter also triggers many birds to migrate to warmer regions for food and to mate.