Inferior goods are those goods for which quantity demanded decreases when income increases. These goods are different from normal goods, for which quantity demanded increases as income increases.

Explanation:
- Income and Demand Relationship: Income has a strong impact on the demand for goods. As income increases, people tend to buy more goods and services, and as income decreases, people tend to buy fewer goods and services.
- Normal Goods: For normal goods, as income increases, the quantity demanded of these goods also increases. For example, if a person's income increases, he may switch from using public transport to using his own car.
- Inferior Goods: For inferior goods, as income increases, the quantity demanded of these goods decreases. For example, if a person's income increases, he may switch from buying generic brand products to buying more expensive branded products.

Types of Inferior Goods:
- Necessities: These are goods that people buy regardless of their income level but may switch to a higher quality or more expensive version when their income increases. Examples include basic food items, clothing, and housing.
- Luxury Inferior Goods: These are goods for which demand decreases as income increases, but they are still considered luxury goods. Examples include second-hand cars, cheap fast food, and budget clothing brands.

Inferior goods are important for understanding consumer behavior and impact on the economy. As people's income increases, they tend to shift their consumption patterns towards higher quality products, which can lead to inflation and changes in market demand.