Italics, based on handwriting script, serve several functions.
They identify the titles of stand-alone creative works like books, films and television series, and paintings. But parts of compositions — chapters, episodes of TV shows, short poems collected into anthologies, and the like — are enclosed in quotation marks.
They denote a word that would be stressed if spoken: “Stop the car — I really have to go to the bathroom.”
They indicate a word being introduced as itself, not as an idea: “Write, right, and rite are all pronounced identically.” Terms of more than one word are often enclosed in quotation marks, but this format may look awkward when used inconsistently alongside single italicized words, so self-referring phrases are often italicized as well (“it’s rank and file, not rank in file”).
They also identify letters used as such: “The letter n on that sign is backward.”
But letters compared to shapes (“turn right at the Y in the road”; “I watched a graceful V of geese fly overhead”) are set in roman type. (The lowercase term roman refers to the default type style.) The same is true for names of letters used in expressions (“dot your i’s and cross your t’s”).
They signal the use of an unfamiliar foreign term: “The Roman legatus was the equivalent of a general in a modern army.”
Note, however, that many words you might think are foreign have been adopted into English, that most welcoming of languages. Check your dictionary’s main section (not the foreign-words appendix); if a foreign term appears there, no italics are necessary. Also, foreign proper nouns need no emphasis.
The rule of thumb for repetition of foreign terms is to italicize on first reference only, and leave them in roman type when they recur. Use your judgment, though, depending on the frequency and interval of recurrence.
Boldface lettering is best reserved for display type (chapter and section titles and the like). But they’re often used in textbooks and other learning materials to emphasize newly introduced terms, such as those that would appear in a glossary or be on a vocabulary quiz. Otherwise, this type style is the printed or posted equivalent of shouting.
3. Quotation Marks
Quotation marks are often used as what are called scare quotes — emphasis markers that communicate novelty, irony, or a nontraditional use of a word or phrase. Writers overuse scare quotes. Except in special cases, they should trust readers to understand the unusual use of a word or phrase.
The context in “I played dumb,” for example, precludes the need for a visual hint to the reader that the writer’s stupidity was an act, but “I had a ‘fit’ so she’d go away” may need a subtle clue that the tantrum was feigned. (Fit appears in single, not double, quotation marks here because they’re used within double quotes.)
Newspapers traditionally omit emphasis because formatting it is time consuming, and many web sites have the same policy, but the many exceptions in both cases — or using quotation marks in place of italics, as often seen on this site — acknowledge that italics and judicious use of boldface and scare quotes aid comprehension. Just don’t have a fit and go “overboard.”