They are names which are general in nature, not specific or unique names. They can be:
names of people - man, woman, child, boy, girl (not Joseph, Jane, Anita, David or Dolly)
names of animals - dog, cat, elephant (not Rex, Rani or Jumbo)
names of places - city, village, river, mountain, country (not Colombo, Cannanore, River Indus or Mount Everest)
names of things - soap, apple, chair, aeroplane (not Lux, Golden Delicious, Neelkamal or Boeing).
From the examples above, you must have understood what common nouns are. Yes...general names. The names given within brackets belong to a different class of nouns. They are proper nouns because they are specific names.
How Important Are Common Nouns?
Whenever we want to say something, we say it about someone or something. That someone or something (called the subject in grammar) will have a noun (or pronoun) as its most important word.
Common nouns are more commonly used than the other type of nouns, namely proper nouns, which are names of unique things.
How Can We Recognize Common Nouns?
The first letter of a common noun is not capitalized except if it is the first word in a sentence.
We bought six chairs.
Chairs are expensive.
What Are Their Types?
Two types:
Countable Nouns (sometimes you will find the name count nouns). These are names of people, places, or things that can be counted (e.g. student, teacher, book, chair, city, village).
Uncountable Nouns (A better name is Mass Nouns - sometimes also called Non-count Nouns). These cannot be counted (e.g. milk, water, furniture, gold, wood, glass, work, information).