Demand is a much broader idea than quantity demanded. Demand refers to a whole range of quantities demanded.
Demand can be defined as the amount of some product that consumers are willing and able to buy over a range of prices. On a graph, demand is represented by a line. Quantity demanded can be defined as the maximum amount that consumers will buy at a given price. It is represented by a point on a graph.
In practical terms, the important thing to remember is that price does not change demand. Price can change the quantity demanded. Demand has changed when consumers are willing to buy more or less of a product when the price has remained the same. Quantity demanded changes when the price changes.