Surface tension is the result of inter-molecular cohesive bonding among the molecules of a liquid. At an interface between the liquid and a gas, the molecules are more attracted to themselves than they are to the gas, so they form a well-defined surface film, which is more stable than the body of liquid beneath it. Inside the body of the liquid, molecules are equally attracted to all other molecules around them, so they tend to slide and jiggle more than at the surface, where molecular bonding forms a barrier between liquid and gas. A better description: http://hyperphysics.phy-astr.gsu.edu/hbase/surten.html
If inter-molecular cohesive bonds are weakened by making the molecules more energetic through the application of heat, for instance, surface tension decreases and liquid molecules can escape through evaporation.
The force of surface tension is directed from molecule to molecule at the surface, where they experience no attraction from gas outside, so they experience less mobility than molecules deeper within the liquid. The film is also attracted to the body of the water, so the surface of a glass of water, for instance, will cup slightly downward in the center, as the film is drawn toward the body of water and away from the gas.