"Is she drinking a glass of water?"
Translation:Ĉu ŝi trinkas glason da akvo?
glaso da akvo = a quantity of water specified by how much the glass can hold glaso de akvo = the glass, a physical object, which happens to have water in it
So when you say "Ŝi trinkas glason da akvo", you're using "glass" as a quantity and are saying that she drank that much water, while if you were to say "Ŝi trinkas glason de akvo", you're using "glass" as a concrete object and saying that she drank the glass. Hopefully vikungen's answer makes more sense now. ;)