"Noun" is the designation for a grammatical concept, a type of word. A noun is generally described as a "person, place, or thing," but it also includes ideas, such as democracy. Verbs designate action words but nouns are the entities that do the actions or have the actions done to them.
There are two types of nouns: common nouns and proper nouns. Common nouns are the kinds of nouns that are in the sentences of this answer, e.g., words such as "noun," "concept," "type," and "word." These are generic. There are many nouns, many concepts, many types of things, and very many words!
Proper nouns, in contrast, designate specific things or ideas. You can have a city (common noun) or you can have "New York," which is a specific city and a proper noun. There can be a person, or you can have "Barak Obama," as a specific person and also a proper noun.
Proper nouns, as can be seen from the examples above, are the type of nouns that are capitalized always.