Wind energy is energy from moving air, caused by temperature (and therefore pressure) differences in the atmosphere. Irradiance from the sun heats 
up the air, forcing the air to rise. Conversely, where temperatures fall, a low pressure zone develops. Winds (i.e. air flows) balance out the differences. Hence, wind energy is solar energy converted into kinetic energy of moving air.
Wind Energy Converters (WECs) - or short: wind turbines - capture the air flow by converting it into a rotational movement, which subsequently drives a conventional generator for electricity.
Wind energy has been used for centuries to pump water and grinding. The industrial breakthrough for the generation of electricity, came in the 1980s.
Today, wind energy is the most mature of the renewable energy technologies apart from hydro. In 2010, the global installation might reach 200GW, up 5GW in 1995 - that equates to an annual growth rate of 27%.