As a new economics student, you may be tempted to use certain terms interchangeably. But they have different meanings in economics speak! Below are some examples that one might come across during their JC Economics journey.
1) Income vs. Money
Money is defined as anything that is widely accepted as a form of payment to facilitate the exchange of goods and services in a market economy. "Money" is a form and representation of value and it has been pegged to many things throughout history. It has evolved from commodities like seashells, salt, precious metals and more importantly gold. In modern times, it is commonly represented by fiat currency (notes printed, issued by the government) and we believe that it stores value because we collectively believe in the government. However, money continues to evolve with the rise of decentralized digital currency - BitCoin. Money should satisfy certain criteria such as being a durable, portable, divisible, widely accepted and scarce commodity that stores value.
Income, on the other hand, is the payment received in exchange for labour effort. "Coincidentally", income is primarily denominated in fiat currency because once again it is the most widely accepted and liquidable form payment of services. Payment can most certainly be in other forms, like gold or stocks but employees may not be so willing to accept such payment methods.
In short, money is a generalized term of fiat currency in today's context while income is the payment of monies in exchange for labour effort. Think of income as a subset of money under a specific context where labour effort is involved.
Do pay attention while using such terms in future. Are you referring to money as a function for exchange or are you referring to it as a payment of labour effort?
2) Quantity Demanded vs. Demand
Let's begin by looking into their respective definitions.
DEMAND
Demand is defined as the willingness and ability to purchase goods and services at various prevailing market prices in a given time frame. In other words: it refers to VARIOUS QUANTITIES being wanted at VARIOUS PRICES.
QUANTITY DEMANDED
Quantity demanded, on the other hand, is the quantity of goods and services demanded at a specific market price. In other words, quantity demanded refers to a SPECIFIC QUANTITY WANTED at a SPECIFIC MARKET PRICE.
Let's further illustrate this with an example:
SCENARIO 1:
"It's a hot day and John demands more ice cream. (seasonal factor)" This causes the demand curve for ice cream to shift rightwards from D0 to D1. Notice the green-highlight on the price axis and the new demand curve. It follows the definition of demand, that at various market prices from 0 to infinity, more quantities of ice cream is wanted. However, the exact quantity is yet to be defined until John knows the prevailing market price of ice cream.
SCENARIO 2:
"It's a cold day and ice-cream sellers decided to reduce the price of ice cream to clear their inventories (seasonal reason). The market price of ice cream fell from $5.00 to $3.00."
Assuming that John's demand curve remained the same at D0. At $5.00, it coincides with the demand curve at point A and the corresponding quantity demanded of ice cream is 10. However, since there is a fall in the price of ice cream, John would now want more quantities of ice cream (obeying the law of demand), moving along the demand curve D0 from point A to B. At $3.00, the quantity demanded of ice cream is 20.
In short, Demand is a nebulous/vague/uncertain indication of the quantity wanted, while Quantity Demanded is a specific indication of the quantity wanted at a specific market price. That is why a non-price determinant causes a change in demand while a price change would cause a change in quantity demanded of the good, ceteris paribus.
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