what is a noun?
a noun is a person, place, thing, or idea
Anonymous is exactly right... Here is a great website on all sorts of nouns with examples for each.
http://www.arts.uottawa.ca/writcent/hypergrammar/nouns.html
To understand what a noun is, you can think of it as a word that names a person, place, thing, or idea. For example, "cat" is a noun because it names a thing. "Mary" is a noun because it names a person. "London" is a noun because it names a place.
If you want further information and examples on different types of nouns, you can visit websites like the one I provided: http://www.arts.uottawa.ca/writcent/hypergrammar/nouns.html. This particular website offers explanations and examples of various types of nouns, such as common nouns, proper nouns, abstract nouns, and more. It can be a helpful resource to deepen your understanding of nouns.