Surface tension is a property of liquid surfaces caused by cohesion. Cohesion is the physical property resulting from the intermolecular forces attracting like-molecules. The molecules on the surface of a liquid have a greater attraction to like-molecules around them than to unlike-molecules.
Molecules on the surface of a liquid experience an inward force balanced by the resistance to compression. Another important point in understanding surface tension is the liquid molecules seek the lowest possible surface area. This is the reason that liquids form droplets on hydrophobic surfaces. The interface of lke-molecules has a lower energy than the interface of unlike-molecules, therefore surface molecules seek to have as many like-molecule interfaces as possible resulting in the lowest surface area.