Plants take in water through their root system by a process called osmosis. The cells that are found in plant roots are designed to have a large surface area which helps facilitate the movement of larger quantities of water across the cell membrane and into the plant.
The movement of water from the surrounding soil and into the plant through the root cells is based on the quantity of water molecules and the types of elements that are dissolved in the water on both sides of the root cell walls. For example, if the soil surrounding the roots is heavily saturated with water and the roots themselves are not, then water will move easily from the soil and into the roots.
Once the water has entered the plant through the root system it is used to move minerals around inside the plant, to cool the plant off as water evaporates from the leaves, to help the plant retain its structure by supporting the cell walls and most importantly, to help feed the plant by driving photosynthesis. If the plant is unable to take in water faster than it loses it then the plant cells lose their turgid, or rigid, quality and begin to collapse. This process is known as wilting.