Proper Nouns and Common Nouns
Proper nouns name specific people, places, things, or ideas.
Since these nouns are naming specific things, they always begin with a capital letter.
Sometimes, they contain two or more important words.
If this is the case, both important words are capitalized, and the whole thing is still considered to be one proper noun even though it's made up of more than one word. How about that?
Common nouns are the opposite of proper nouns. They are your run of the mill, generic nouns. They name people, places, things or ideas that are not specific.
Since these nouns are not naming anything specific, they do not need to start with a capital letter unless they begin a sentence.
Every proper noun has a common noun equivalent, but not every common noun has a proper noun equivalent!
For example, dust is only a common noun. There is no specific kind of dust, so it's just common.