Nouns refer to people, places, things, and ideas. There are two main types of nouns: countable nouns and uncountable nouns. Countable nouns refer to objects that can be counted, like books or cars, while uncountable nouns refer to substances or abstract concepts that cannot be counted, like furniture or advice. It's important to distinguish between countable and uncountable nouns because their usage is different, particularly regarding determiners like "a" and "some" and verbs like "is" and "are".