* Common nouns refer to general categories of people, places, things, or ideas. They are not capitalized unless they start a sentence.
* Proper nouns refer to specific people, places, things, or ideas. They are always capitalized.
Examples:
* Common noun: ninja (a type of warrior)
* Proper noun: Leonardo (a specific ninja turtle)
Since "ninja" refers to a general type of warrior, it is a common noun.