demand curve
A demand curve is simply a picture of the law of demand. Imagine asking everyone in town, "At each possible price, how many cups of coffee would you buy in a week?" At 5 dollars they'd buy few; at 4 a bit more; at 2 a lot. Plot price up the side and quantity along the bottom, join the dots, and you get a line that slides downward from upper-left to lower-right. That downward slope is the law of demand made visible: higher price, smaller quantity bought.
Read it either direction and it tells you something useful. Pick a price and slide across to read off how much people will buy at that price. Or pick a quantity and read up to find the highest price at which that last unit can be sold — which doubles as a rough measure of how much that buyer values it (their willingness to pay). A demand curve always belongs to a particular thing, a particular market, and a particular stretch of time, with everything else held equal: same incomes, same tastes, same prices of other goods. Strictly, the line maps quantity demanded against price and nothing else.
The crucial discipline is knowing what moves you along the curve versus what moves the whole curve. A change in the good's own price slides you up or down the existing line — economists call that a change in quantity demanded. But if buyers' incomes, tastes, expectations, the price of a substitute, or the number of buyers change, the entire curve jumps left (less wanted at every price) or right (more wanted at every price). That whole-curve jump is a change in demand. Keeping these two apart is the foundation of analysing any market shock correctly.
A bakery's demand curve might say: at 4 dollars a loaf, 50 sold a day; at 3 dollars, 80; at 2 dollars, 120. Drop the price and slide down the curve to a bigger quantity — but a new café opening across the street shifts the whole curve left.
Moving down the same curve is a price effect; jumping to a new curve is everything else.
By convention economists put price on the vertical axis and quantity on the horizontal — backwards from the usual maths habit of putting the thing you control on the bottom. Blame Alfred Marshall; the convention stuck, and it trips up almost everyone at first.