Surface tension is define as a force by virtue of which a liquid surface tries to minimize its surface area.It's unit is N/m
One can say surface tension is a tensile force which is acting at the boundary of a liquid and air or between two immisible liquid
An eg. of surface tension is a liquid drop.
In case of liquid drop, the surface tension acting inward towards the center of liquid drop and we know that in case of spherical surface a physical quantity have it's minimum surface area.So,this surface tension force tries to minimize surface area of the liquid drop and as a result we get a spherical shape of liquid drop.