Surface tension is an effect where a liquid is high on the surface. Because of this, some insects (e.g. water striders) can run on water surface. This property is caused by the molecules in the liquid being attracted to each other (cohesion), and is responsible for many of the behaviors of liquids.
The tension of the surface film of the liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid which tends to minimize surface area.