Can you explain the difference between a change in quantity demanded, and a change in demand?

If you are asked to explain, or identify, a quantity demanded, it will be a singular point that represents a quantity consumers are prepared to buy at a particular price on a given demand curve, or in a demand schedule. The determinant of the quantity demanded is, in this case only, price.

However, you may have read that price is subject to change; it is not constant. There are factors that you need to know which follow on a price change. One is the substitution effect. If the price of your apartment is hiked by 20 percent, what are you likely to do? You will likely investigate the price of a shared unit, or a room in a house. We can say that, as the opportunity cost of your apartment unit rises, there are many uses you can make of that extra 20 percent of rent, and you will be very likely to buy some substitute housing service.
Another implication of a price increase arises when your income stays fixed. If prices generally rise, you will have to buy less of everything: books, food, or travel possibilities. This is the income effect at work. Given major price increases, you will buy less of all goods and services.

A price change causes a change in quantity demanded, but a change in demand means that the entire demand curve has shifted. A shift to the right denotes an increase, while a shift to the left indicates a decrease. In the first case, a shift to the right of the demand curve, we have more quantity demanded at all price levels. In the second scenario, a shift to the left of the demand curve, we have a lower quantity of goods demanded at all price levels.