The indefinite articles a and an are used to modify singular nouns. When using a plural noun, these two articles are unnecessary. Plural nouns can take either a definite article or no article at all.
The definite article is the word the. It precedes a noun when something specific (i.e., definite) is being referred to.
Indefinite articles, on the other hand, are used before nouns that are nonspecific within their class.