A noun that gives a general or common name to persons, places, animals or things is called a common noun. It names any person, place, animal or thing. It is the name of a group of similar things. A common noun is generally not written in capital letters.
Person - Boy
The word boy is a general word. It could refer to any boy and not to any particular or special boy. The word boy is a common noun.
Place - City
The word city refers to any city. It is a general word for city and does not refer to any one special city. The word city is a common noun.
Animal - Horse
The word horse refers to the animal 'horse' and not to any special horse. It is not the special name of any one particular horse. The word horse is a common noun.
Thing - Chair
The word chair refers to any chair and not to any one special chair. The word chair is a common noun.