Summer is the hottest of the four seasons (winter, spring, summer, autumn). It occurs between spring and autumn (or fall). Summer is also the season during which the amount of time that is light out is longest. As summer progresses, the days continue to get shorter through autumn, and the days become shortest during winter.
Based on the meteorological definition of summer, summer occurs from June 1st through August 31st in the Northern Hemisphere and December 1st through February 28th in the Southern Hemisphere. Based on the astronomical definition, the dates vary from year to year.
Summer is also the season during which many schools and universities have extended breaks to take advantage of the warmer weather and longer days. The dates vary significantly based on the actual institution as well as the location, but in some countries, such as the United States, summer vacation can last upwards of 2-3 months. In the Southern Hemisphere, summer vacation typically coincides with major holidays such as Christmas and New Year's Day.
What causes summer?
Summer, and the seasons in general, are caused by the fact that the axis of rotation of the Earth is tilted (~23°) relative to the plane formed by the orbit of the Earth around the Sun. As a result, different parts of the Earth receive varying amounts of sunlight throughout the year.
Since the Northern and Southern Hemispheres face the Sun at opposite times throughout the year, their seasons in turn occur at opposite times; when the Northern Hemisphere faces the Sun, it is summer in the Northern hemisphere and winter in the Southern Hemisphere. Below is a figure depicting the tilt of the Earth relative to its orbital plane.
The regions of the Earth that experience the four seasons are referred to as temperate regions. Those that do not are referred to as tropical regions. The reason that tropical regions do not experience the four seasons as temperate regions do is because they experience far less variation in sunlight (and therefore temperature) throughout the year because of their more constant position relative to the Sun. Instead, these regions experience a dry and a wet season. The wet season is a period of time each year during which a tropical region receives most of its average annual rainfall. Tropical regions generally remain relatively hot year round, so in a way, experience summer all year, with the only difference being whether it is wet or dry.