The name of a group of persons or objects of the same kind considered as a unit. As the name suggests, a common noun is a noun. Examples of common nouns are people, cities, and planets.
The opposite of a common noun is a proper noun. One easy way to differentiate them is that proper nouns are the only nouns in which the first letter must be capitalized, regardless of where in the sentence it falls. Another difference between them is that a proper noun is specific, like John Doe or the car brand Toyota.