Summer time, known in North America as the Daylight Saving Time (DST), is a system that is intended to "save daylight" in the summer. Because of that in Europe the system is known as "Summer Time".
Official time moved forward (usually) one hour earlier than the official time zone and enforced during the spring and summer. It is intended to further adjust the hours while it was still bright with hours of work and school activities. DST is normally used in areas where the temperature is, because quite a big difference between the light than the dark days of the season in these areas.