This document defines and provides examples of different types of articles in English grammar:
1. The definite article "the" is used to refer to specific nouns and known things. It can also be used with proper nouns referring to one-of-a-kind things like the sun.
2. The indefinite articles "a" and "an" are used for non-specific or unknown nouns. "A" is used before consonant sounds and "an" is used before vowel sounds.
3. There is also a "zero article" where no article is used, like with generalizations or with days, months, and academic subjects.