Weather is the state, or condition, of the atmosphere at any time.
It is popularly thought of in terms of temperature, precipitation (if any), cloud cover, and wind speed. (Because of this, words like hot, cloudy, sunny, rainy, windy, and cold, are often used to describe it.)
Weather is caused by uneven heating of Earth's surface.
Energy from the sun heats Earth's surface. But because our planet is shaped like a sphere, this energy isn't absorbed equally everywhere on Earth.
Regardless of season, the sun's rays always strike most directly near the equator, which keeps temperatures there higher than anywhere else on Earth. At latitudes farther from the equator, sunlight strikes the surface at lower angles--that is, the same amount of solar energy that strikes near the equator strikes here too, but is spread over a much greater surface area. As a result, these locations are heated less intensely than those near the equator. It is this temperature difference that drives air to move around the globe, giving us weather.
So you can think of weather as the atmosphere's way of moving heat from one part of the world to another in an attempt to balance it out. However, because of how the Earth heats up (as we've just learned above), the atmosphere's work is never done--which is why we're never without weather.
How Weather Differs From Climate
Unlike climate, weather has to do with short-term (on the scale of hours to days ahead) variations of the atmosphere's behavior, as well as how these affect life and human activities "in the now".