One simple equation:
SUGAR + YEAST = ALCOHOL + CO2 + HEAT
What can we learn from this?
"Riper" grapes have juice with higher sugar levels. If the juice is fully fermented (all sugar converts to alcohol) to make a dry wine, the wine will have a higher percentage of alcohol.
Sometimes the wine is not fully fermented (riesling for example) and will have a lower alcohol level and, what's called, residual sugar. These wines will taste a bit sweet.