A noun is any word that can form the head word in a noun phrase or be the subject or object of a verb . Semantically speaking, a noun is any word that 'labels' or 'names' a person, thing or idea.
There are several types of noun: common noun (e.g. computer, sandwich, cats), proper noun (proper nouns are names for individual nouns, e.g. Coke, London, Simon), abstract noun (abstract nouns are 'ideas', e.g. death, hunger, beauty), concrete nouns (concrete nouns are solid objects in the real or imaginary world, e.g. bread, butter, clock) collective nouns (collective nouns name groups of individual or things, e.g. parliament, audience collective nouns are often treated as if they were singular, e.g. 'The choir is singing well.'), mass (or non-count ) nouns (mass nouns exist as an undifferentiated mass, e.g. card, beer, milk, cake), and count nouns (count nouns exist as countable items, e.g. bottle, pencil).