Summarize:

In simplest terms, this means that any word you would underline in a handwritten letter to emphasize, or would say more forcefully than others when speaking, you would italicize in a word processing document or on a website. For example, "I am the only living only child among my first cousins." Foreign words and phrases used in English writing for effect are italicized, such as using the German Turnverein in place of the common American term "athletic club." Foreign words that have become part of the English language through common usage, such as "kindergarten," are not italicized. The Latin words for the genus and species of an organism are also italicized, as in Homo sapiens. This is normally done when the terms are first introduced, particularly if they have a meaning different from what the reader is used to. Physical constants, such as c for the speed of light, and variables in algebra, as in " n = 2," are also rendered in italics. A block quotation is a lengthy quotation (typically 100 words or more, or at least 5 to 8 lines of text) set off from the rest of the text and indented. Block quotations are often rendered in italics, or in a different font or point size. When an item in a block quote would be italicized, and the rest of the block quote is already in italics, the item is commonly rendered in normal text to set it off from the block quote. Large blocks of italicized text can be hard to read on some computer screens. In such cases, you may want to render block quotes in a different font than the surrounding text. While you do not italicize the make, model, or military designation of a vehicle, vessel, or craft, you do italicize the following named vessels: Trains (The Golden State Limited), but not the names of individual cars. Ships, either military or passenger vessels (USS Lexington, Queen Elizabeth II). Aircraft names or nicknames not based on construction or performance characteristics (Memphis Belle or Cutter's Goose from the TV show Tales of the Gold Monkey, but not the Batplane). Spacecraft, either real or fictional (the Space Shuttle Challenger, the starship Enterprise, the Millennium Falcon). Space missions, such as Apollo 11, are not italicized. The following works have their titles rendered in italics, unless a particular style manual (such as AP or MLA) dictates otherwise: Books (Harry Potter and the Sorcerer's Stone), except for the titles of religious books such as the Bible or the Koran. Titles of chapters, sections, and short stories within anthologies are enclosed in double quotes. Magazines (Collier's, Reader's Digest). Titles of articles ("I Am Joe's Kidney") are enclosed in double quotes. Newspapers (USA Today, The Wall Street Journal). Plays (Romeo and Juliet, Who's Afraid of Virginia Woolf?). Court cases (Brown v. Board of Education of Topeka). Television and radio programs (Star Trek, The Shadow). Titles of episodes ("Amok Time," "Temple Bells of Neban") are enclosed in double quotes. Record albums (Mandatory Fun, Red). Titles of album tracks ("Word Crimes," "I Knew You Were Trouble") are enclosed in double quotes. Artwork (Mona Lisa, The Last Supper). Punctuation marks that are part of the title are italicized with the rest of the title. In fiction, when a character's thoughts are put into words for the sake of the reader, they are commonly rendered in italics, as in "Kate regarded her husband with apprehension. That's funny, Joe never asks for a second cup of my coffee." If you're attempting to render a sound with a word the way the reader would hear it, italicize the word: "The startled cat gave out an angry Meeeowwwrr!" If you're using a word to describe a general sound, you don't italicize it: "The cat meowed."
Italicize words you want to give particular emphasis to. Italicize foreign words that have not been fully adopted into English. Italicize technical terms. Italicize block quotations. Italicize the names of named major transportation vehicles. Italicize the titles of certain major creative works. Italicize a character's internal dialog. Italicize onomatopoeia (word sounds).