Weather is the day-to-day state of the atmosphere, including temperature, humidity, wind, and precipitation. It's driven mainly by energy from the Sun, which heats different parts of Earth's surface unevenly because of the planet's tilt and curved shape.
As the Sun warms the air near the ground, that warm air rises and cooler air rushes in to take its place, creating wind. Differences in air pressure between regions push air masses around the globe, and where warm and cold air masses meet, fronts form that often bring changes in weather, including storms.
Water plays a huge role too. As the Sun heats oceans and lakes, water evaporates into the air, and when that moist air cools and rises, the water vapor condenses into clouds, eventually falling back to Earth as rain, snow, or other forms of precipitation.