Economics is all about scarcity, and scarcity drives pricing. Most people have a general understanding of the interplay of supply and demand without really knowing how it works. As one half of the supply/demand interplay, the law of demand states that the quantity demanded rises as price falls. This takes into account two items, the substitution effect, which indicates price changes relative to another good, and the income effect, which says that the change in price changes disposable income.
If the demand curve is linear, the relationship can be estimated with the demand curve formula Qd=a-b(P), whereas Qd is the quantity demanded in number of units, a is a constant which represents the demand determinants (this is the X intercept), b is the relationship between Qd and price, which is a negative (inverse slope of the demand curve), and P is price.
The demand curve slopes downward on a graph. This is because consumers are always willing to buy more goods at a lower price. Typically, demand shifts from demographics, prices, income, and consumer tastes and trends. Firms with market power can also impact demand with advertising.
Keep in mind, this formula is only used as an estimation of demand, as real-world applications are not linear.