* Pronouns replace nouns. They stand in for a noun that has already been mentioned, making it unnecessary to repeat the noun. Examples: "I," "you," "he," "she," "it," "we," "they," "this," "that," etc.
* Common nouns are general names for people, places, or things. Examples: "dog," "city," "building."
* Proper nouns are specific names for people, places, or things. They are always capitalized. Examples: "Fido," "London," "Empire State Building."
Since pronouns don't name things directly, they don't fit into the categories of common or proper nouns.