Quantity demanded stands literally for itself - it's the quantity demanded by economic agents (consumers, firms, etc.). It's a single number. Demand, on the other hand, is a function of quantity and price. It states how much will be demanded at any price. It can be line or curve. It's mathematical concept, it does not really have nothing to do with "real world". It's not a number (like quantity demanded: 157 apples, 1,5million cars, etc.), but a math function (i.e. P=8-3Q or something similar).

Demand vs Quantity Demanded:

Demand and quantity demanded are both important concepts in economics that relate to the consumer's desire and ability to purchase a certain quantity of a product or service at a given price. While they may seem similar, they have distinct differences that are crucial to understanding how markets function.

1. Demand:
Demand refers to the overall desire and willingness of consumers to buy a particular product or service at various price levels during a specific period. It represents the entire demand curve, which shows the relationship between the price of a product and the quantity of that product consumers are willing and able to purchase. Demand is influenced by various factors, including:

- Price: As the price of a product increases, demand generally decreases, assuming other factors remain constant.
- Income: Higher income levels generally lead to increased demand for most goods and services.
- Tastes and preferences: Changes in consumer preferences can significantly impact demand.
- Price of related goods: The demand for a product can be influenced by the prices of substitute goods or complementary goods.
- Population: The size and composition of the population can affect the overall demand for goods and services.

2. Quantity Demanded:
Quantity demanded refers to the specific quantity of a product or service that consumers are willing and able to purchase at a particular price point. It represents a single point on the demand curve and can change as the price changes. Quantity demanded is influenced by the following factors:

- Price: A change in price directly affects the quantity demanded. When the price decreases, the quantity demanded generally increases, and vice versa, assuming other factors remain constant.
- Other factors: While the price is the primary determinant of quantity demanded, other factors, such as income, preferences, and availability of substitutes, can also influence the quantity demanded at a specific price.

Key Differences:
The main differences between demand and quantity demanded can be summarized as follows:

1. Scope: Demand represents the entire demand curve, reflecting all possible price-quantity combinations, while quantity demanded refers to a specific quantity demanded at a given price.

2. Factors: Demand is influenced by various factors like price, income, preferences, and population, whereas quantity demanded is primarily influenced by the price of the product or service.

3. Representation: Demand is usually represented graphically as a demand curve, whereas quantity demanded is represented as a point on that curve.

4. Change: Demand can change due to shifts in the demand curve caused by changes in factors affecting demand, while quantity demanded changes along the same demand curve due to changes in price.

In summary, demand represents the overall relationship between price and quantity, influenced by various factors, while quantity demanded refers to a specific quantity consumers are willing to purchase at a given price. Understanding these concepts is essential for analyzing market behavior and making informed economic decisions.