Common nouns and proper nouns are types of noun used every time. Many do not know the differences between the two.
Common noun refers to a type of noun that states places, persons or things in general terms. Example - man, city or country. In a sentence. 'The man is tall, the city is beautiful'.
A proper noun refers is a type of noun that is specific on a person, place or thing. It gives a name to the noun.
Example - Sarah, London, Australia. In sentences. 'Mr Sam is tall, London is beautiful, Sarah is my first child'.
The only differences between the two is that one refers to things in general terms and the other is more specific about things.