In titles, the words that are not typically capitalized are (A.) articles.
Although there are different style guides that indicate how to write titles, most of them coincide that articles should not be capitalized, unless an article is the first word in a title. An article is a word that is used before a noun to indicate if the noun refers to a specific entity or an unspecific one. There are indefinite articles (a/an) and a definite article (the) and they can be used with singular and plural nouns.