What is wine (yes, we actually had at least one customer debate this)?

"Wine" as per 27CFR24 (code of federal regulations) "when used without qualification, the term includes every kind (class and type) of product produced on bonded wine premises from grapes, other fruit (including berries), or other suitable agricultural products and containing not more than 24 percent of alcohol by volume."

Why is the grape sugar level important with wines?

Yeast converts sugar to alcohol. Roughly 2% sugar will yield 1% alcohol. An example, grapes with 24% sugar level will produce wine with approximately 12% alcohol. The riper the fruit, the higher the sugar level and the lower the acid or tartness level.