What are water droplets found on a car in the early morning called?

When water condenses on a surface, it is commonly referred to as dew or condensate. 

When you leave a car outside, there is moisture in the air inside the vehicle. This air tends to be around 70 degrees if an ac unit is running, whereas the exterior air is a different temperature. If the air outside the car is colder than the air inside, then the windows will conduct the heat from the air inside into the air outside, causing water to condense on the windows.

This can happen in other places, too. Spiderwebs can catch dew on their strands, while tabletops and decks can accumulate puddles. Even bridges can gather dew. When the air drops below freezing, ice can form.