Common nouns are general names. They name any person, animal, place, thing, or idea. Do not use a capital letter for a common noun, unless it starts a sentence.
Look at the picture above. The word queen is a common noun, a general word for any queen in any kingdom.
Common nouns contrast with proper nouns, which are specific names.
Proper nouns are unique names. They name specific people, animals, places, things, or ideas. They always begin with capital letters.
Look at the picture above. The word Snow Queen is a proper noun. It is the name of a specific queen, it is her own name. But the word queen is a common noun because it could refer to any queen in the world.