Tears or Legs on a Wine glasses

“Tears” or “Legs" in a wine glass are caused by the Marangoni effect, a fluid dynamics phenomenon driven by a surface tension gradient between the water and alcohol in the wine.

As alcohol evaporates faster than water from a thin film on the glass, the remaining liquid becomes more watery and has higher surface tension, which pulls more liquid upwards before gravity causes droplets to fall back down as visible "tears" or "legs".
When you swirl a glass of wine, it coats the sides of the glass with a thin film of wine. Alcohol has a higher vapour pressure than water, so it evaporates more quickly from the thin film on the glass's surface. 
As the alcohol evaporates, the concentration of alcohol in the film decreases making the film's surface tension higher than that of wine in the glass. The higher surface tension of the wine on the glass pulls the liquid film upwards, creating a flow along the surface - the Marangoni Flow. The liquid accumulates at the top of the film until gravity overcomes the surface tension force, causing the accumulated liquid to form droplets that fall back into the glass. 
The higher the alcohol content,more alcohol evaporation, creating a stronger surface tension gradient and more pronounced legs.
Sweeter wines often have a slower descent of droplets due to the viscosity of the sugar.
And lastly residue from cleaning the glass, soap etc can affect surface tension and thus affects the leg formation.
The term tears of wine was first coined in 1865 by physicist James Thomson, the brother of Lord Kelvin. The effect was named after Italian physicist Carlo Marangoni who later studied the topic for his doctoral research and published his findings in 1865.