Detergents are synthetic chemical salts which change the surface property of water and which are used as cleaning agents. Detergents contain one or more chemicals called surfactants, which reduce the surface tension of water by weakening the forces between water molecules. This enhances the cleaning action of water. A detergent molecule has two different parts. One part is hydrophilic (water-loving head) while the other is hydrophobic (water-hating tail). When soiled clothes are put in a detergent solution, water-loving head remains in water while the tail attaches to the grease or oil holding the dirt particles on the soiled material. Rubbing helps in breaking up the dirt. In this way the dirt particles are dislodged from the surface of the material and drawn into the wash water. After the dirt particles are suspended in water, a thin layer of surfactant remains around the particles keeping them separated. This prevents the dirt from settling on the washed material again. This is how detergents help in cleaning.
The common soap (sodium salt of fatty acid) is one of the oldest detergents but it is relatively weak. Synthetic detergents are stronger than soap and give lather even in hard water.