What is surface tension and how does it cause capillary action?

Surface tension is the property of liquid surfaces to behave like elastic membranes due to cohesive forces between surface molecules. It is measured as force per unit length (N/m). Capillary action occurs due to the interplay between adhesive forces (liquid-solid) and cohesive forces (liquid-liquid). When adhesive forces dominate (water in glass tube), liquid rises; when cohesive forces dominate (mercury in glass), liquid depresses. Capillary rise is h = 4σcosθ/(ρgd), where σ is surface tension, θ is contact angle, and d is tube diameter.