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.

Proper and common nouns can perform any of the noun jobs (subject, direct object, indirect object, object of the preposition, predicate nominative). Here are some examples of all of these noun jobs:

Subject: The students dutifully studied grammar. 

Direct Object: They engaged their minds and learned quickly.