What type of word is "All"? Article, possessive, indefinite, or proper?

"All" is a determiner: Distributive determiners.

Explanation:

"Determiners" are words that come prior to a noun. Like, in the sentence, 'A' dog is barking. Here A is a determiner before the noun 'dog'. All articles, possessive pronouns like "my, your, his, her" and numbers like one, ten are determiners. Distributives like all, half, both are also determiners.

Articles are "a, an and the". When we want to refer to specific noun like Taj Mahal, we use ''the". It is called definite article. In case of unspecific nouns like apple, mango, table, we use a or an. "An" is used before "vowels" (a, e, i, o, u). This is called indefinite article.