The sentence "Julie veut du chocolat." can be literally translated as, "Julie wants of the chocolate." This obviously doesn't work in English very well. In English you would say, "Julie wants some chocolate.", meaning she wants a part of some indeterminate amount of chocolate. Equally possible is the English translation, "Julie wants chocolate.", without the "some". In both cases it is a part of some indeterminate amount of chocolate.

However, if you happen to have some sweets in your pocket as, e.g., some pieces of chocolate, you could say, "Tu veux des chocolats?", which is a different question from "tu veux du chocolate?".

Tu veux des chocolats? -- Do you want some sweets?
Tu veux du chocolate? -- Do you want (some) chocolate?

I hope this helps, -- Chris (not a native speaker).

Thanks so much for your answer. That is bit somewhat clearer I guess but still a little confusing. I am sure it will make more sense the more I know. The problem is not being in a sutuation where you do not use the language day to day and don’t get the inscand outs of the language.