Title case or headline case is a style of capitalization used for rendering the titles of published works or works of art in English. When using title case, all words are capitalized, except for minor words (typically articles, short prepositions, and some conjunctions) that are not the first or last word of the title. There are different rules for which words are major, and hence capitalized. As an example, a headline might be written: "The Quick Brown Fox Jumps over the Lazy Dog".
Rules
Rules of title case are standardized only at the level of house styles and individual style guides, not universally. Most English style guides agree that the first and last words should always be capitalized, whereas articles, short prepositions, and some conjunctions should not be. Other rules about the capitalization vary.
In text processing, title case usually involves the capitalization of all words irrespective of their part of speech. This simplified variant of title case is also known as "start case" or "initial caps".
Capitalize the first word of the title/heading and of any subtitle/subheading
Capitalize all major words (nouns, verbs including phrasal verbs such as "play with", adjectives, adverbs, and pronouns) in the title/heading, including the second part of hyphenated major words (e.g., Self-Report not Self-report)
Capitalize all words of four letters or more.
Lowercase the second word after a hyphenated prefix (e.g., Mid-, Anti-, Super-, etc.) in compound modifiers (e.g., Mid-year, Anti-hero, etc.).
Capitalize prepositions and conjunctions of four letters or more.
Lowercase the articles the, a, and an.
Capitalize the first and last words (overrides the rules above).
Capitalize the "to" in infinitives (e.g., I Want To Play Guitar).
The Bluebook
According to the 21st edition of The Bluebook, written by a consortium of U.S. publishers for citing legal publications, the following title capitalization rules should be applied:
Capitalize the first and the last word.
Capitalize nouns, pronouns, adjectives, verbs (including phrasal verbs such as "play with"), adverbs, and subordinate conjunctions.
Lowercase articles (a, an, the), coordinating conjunctions, and prepositions of four letters or fewer.
Lowercase "to" in infinitives (though not defined in the stylebook).
Nouns, verbs, and adjectives (except possessives) are usually capitalized.
Pronouns and adverbs may or may not be capitalized.
Articles, conjunctions, and prepositions are normally not capitalized.
However, judgement is allowed based on factors such as sense, emphasis, structure, and length when deciding which words to capitalize.
MLA Handbook
According to the 9th edition of the MLA Handbook, published by the Modern Language Association of the U.S., the following title capitalization rules should be applied:
Capitalize the first word of the title/heading and of any subtitle/subheading.
Capitalize all major words (nouns, verbs including phrasal verbs such as "play with", adjectives, adverbs, and pronouns) in the title/heading, including the second part of hyphenated major words (e.g., Self-Report not Self-report).
Lowercase the second word after a hyphenated prefix (e.g., Mid-, Anti-, Super-, etc.) in compound modifiers (e.g., Mid-year, Anti-hero, etc.).
Do not capitalize articles, prepositions (regardless of length), and coordinating conjunctions.
Do not capitalize "to" in infinitives (e.g., I Want to Play Guitar).
Title case in references
The use of title case or sentence case in the references of scholarly publications is determined by the citation style used and can differ from the usage in title or headings. For example, APA style uses sentence case for the title of the cited work in the list of references, but it uses title case for the title of the current publication (or for the title of a publication if it is mentioned in the text instead). Moreover, it uses title case for the title of periodicals even in the references. Other citation styles like Chicago Manual of Style use title case also for the title of cited works in the list of references.