This constant circulation of water from the earth to the atmosphere and back again is called the hydrologic cycle. The hydrologic cycle is very complicated. Some of the water which condenses into clouds and precipitates actually evaporates before reaching the Earth's surface. This same vegetation can also place water vapor into the atmosphere via a process called transpiration.
Humans can place water vapor into the atmosphere by perspiration or by simply exhaling.
If all of the vapor in the atmosphere were to suddenly condense and fall out as rain, it would cover the entire Earth with one inch of water.