The troposphere is the lowest part of Earth's atmosphere. It contains about 75% of the atmosphere's mass and 99% of its water vapor and aerosols. Its height is about 15 km. It is taller in the tropics (20 km) and shorter at the poles (7 km).
The troposphere is where Earth's climate like rain, snow, thunder, or storms occur. Clouds can form up to a height of 10-15 km. The troposphere is where people live, because it reaches down to ground level. The next layer up is called the stratosphere. Between the two layers there is the tropopause.
In the troposphere, the temperature goes down as the altitude increases. This is different from the stratosphere. This also means that the troposphere is quite unstable: gases can easily rise up or fall down. Hence the troposphere is well mixed. It is this property that causes the general atmospheric circulation.