Demand in economics

Market Demand vs. Aggregate Demand
The market for each good in an economy faces a different set of circumstances, which vary in type and degree. In macroeconomics, we can also look at aggregate demand in an economy. Aggregate demand refers to the total demand by all consumers for all good and services in an economy across all the markets for individual goods. Because aggregate includes all goods in an economy, it is not sensitive to competition or substitution between different goods or changes in consumer preferences between various goods in the same way that demand in individual good markets can be.
Macroeconomic Policy and Demand
Fiscal and monetary authorities, such as the Federal Reserve, devote much of their macroeconomic policy making toward managing aggregate demand. If the Fed wants to reduce demand, it will raise prices by curtailing the growth of the supply of money and credit and increasing interest rates. Conversely, the Fed can lower interest rates and increase the supply of money in the system, therefore increasing demand.1 In this case, consumers and businesses have more money to spend. But in certain cases, even the Fed can’t fuel demand. When unemployment is on the rise, people may still not be able to afford to spend or take on cheaper debt, even with low interest rates.
Determinants of Demand
There are five determinants of demand. The most important is the price of the good or service itself. The second is the price of related products, whether they are substitutes or complementary.
Circumstances drive the next three determinants. The first is consumer incomes or how much money they have to spend. The second is buyers' tastes or preferences in what they want to purchase. If they prefer electric vehicles to save on gasoline, then demand for Humvees will drop. The third is their expectations about whether the price will go up. If they are concerned about future inflation they will stock up now, thus driving current demand.
Law of Demand
The law of demand governs the relationship between the quantity demanded and the price. This economic principle describes something you already intuitively know. If the price increases, people buy less. The reverse is also true. If the price drops, people buy more.
But price is not the only determining factor. The law of demand is only true if all other determinants don't change.
In economics, this is called ceteris paribus. The law of demand formally states that, ceteris paribus, the quantity demanded for a good or service is inversely related to the price.