Demand is one of the two fundamental forces driving every market (the other being supply). It represents how much of something consumers want to buy at different prices. The key phrase: willing AND able — wanting a Ferrari is not demand unless you can also afford one.
The law of demand states: when prices go up, demand goes down (and vice versa), all else being equal. If coffee costs $2, you might buy 5 cups a week. At $5, maybe 2 cups. At $10, you switch to tea. This inverse relationship creates the downward-sloping demand curve.
Factors that shift demand: income changes, population growth, consumer preferences, prices of substitutes/complements, and expectations. When the iPhone launches a new model, demand shifts right (increases) as consumers rush to buy. When gas prices spike, demand for electric vehicles shifts right as consumers look for alternatives.