The Water Cycle

The water cycle is the process by which water travels.  The water in the universe is the same all of the time.

Definitions
Evaporation-Water is transferred from the sea or a lake into the sky by the heat from the sun.
Condensation-The evaporated water cools at dew point and forms clouds.
Precipitation-Things which come from clouds, usually rain but can be snow, hail etc.
Transpiration-Trees give off water.
Run-Off-Water runs down the land to the sea, this can sometimes cause flash floods.
Percolation/Infiltration-water soaks into the ground.
Groundwater Flow-Water travels underneath the ground.

Differ According to Seasons
In the winter there is less evaporation due to the sun being low in the sky, there is therefor less rain.  The ground is harder so less percolation can occur, lading to more surface run-off and therefor more flooding.

Affect From Humans
Deforestation means that less transpiration occurs, also contributing to less rain.  Humans also build tarmac which means less percolation and more run-off, causing flooding.  There is also less interception (when trees stop water hitting the ground), which can also cause flooding.  Global warming means more evaporation so therefor more precipitation.