Law of Demand
As the price of a good or service increases, the quantity demanded of the good or service will decrease and vice versa.
The reason the demand curve almost always follows this downward trend is that as the price decreases people have a lower opportunity cost (as they will be able to spend/invest that extra money elsewhere). This is fairly simply thought when looking at how many rational consumer think when buying. Lets say a person wants a certain new computer, but this computer costs $1200, this is more than the person is willing to pay. If the price drops enough, this person is more willing to buy the computer. In a group setting, a drop in price that would make even one consumer want to buy that good. That would make the demand curve have one more quantity demanded. When the demand curve is sloping up instead of sloping down it is a result of the good being a Veblen good, a Giffen good or because of the role of expectations.
Note: Mr. Ormsby didn't make it clear, but there is a difference between demand, and quantity demanded. The law of demand refers to quantity demanded rather than demand. An increase in demand refers to a shift of the actual demand curve to the right.