Wine legs and tears are caused by wine dribbling down the sides of a wine glass. They are most often visible after tasting a wine, when the glass has been tilted on its side, and then righted.
Some of the wine will have coated the inner sides of the glass, and will begin dripping back down into the bottom. This is caused by alcohol in the wine evaporating, breaking the surface tension on the upper edge of the wine, causing the watery components to fall away back into the glass.
The shape and size of the legs are a sign of the wine’s viscosity (which in turn is affected by the amount of glycerols and alcohol found in the wine).
A wine with low viscosity will have smaller, more watery legs. A wine with higher viscosity will have larger, slower legs.