The best way to distinguish between these two is to understand that demand is represented as a line on a graph while quantity demanded is represented by a point.
Demand is the amount of a good or service that consumers will be willing and able to buy at any given price. That means that it is graphed as a line when the two axes of the graph are price and quantity. The line shows the quantity of the good or service that people will buy at each price level.
Quantity demanded is the amount of the good or service that consumers are willing to buy at a specific price. This means that it is only one point on the demand curve. There will be a different quantity demanded at every point on a given demand curve, meaning that for the same level of demand, there are many quantities demanded.