Daylight saving time (also called DST) is the North American term for a system intended to "save" daylight (the British observe summer time, and likewise the Europeans). The official time is adjusted forward, (usually) one hour from its official standard time, remaining that way for the duration of the spring and summer months. This is intended to provide a better match between the hours of daylight and the active hours of work and school. DST is most commonly used in temperate regions, due to the considerable variation in the amount of daylight versus darkness through the seasons in those regions.
Note that the term commonly used in the United States, Daylight Savings Time, is technically incorrect: the correct name as provided by the act which inaugurated it in the United States is Daylight Saving Time.