The water is condensation that forms on the outside of the glass.
With a cold drink, it will cool down the air immediately surrounding the glass. One effect of temperature on air is that it changes how much moisture that air can hold – how humid it can be. So as the air cools, the moisture that was being held in the air no longer can be, and it condenses – on a large scale this is how fog is formed (which then burns off as the air temperature warms up), and on a smaller glass scale, the water condenses on the outside of the glass, runs down it and forms a ring.
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