Titles
Italics can be used to indicate the title of stand-alone things, such as books ("To Kill a Mockingbird"), magazines ("Time"), TV and radio shows ("60 Minutes"), movies ("Gods and Generals"), plays: ("Twelve Angry Men") and famous speeches (the "Gettysburg Address"). Do not italicize the titles of poems unless they are long enough to be published by themselves, such as "Paradise Lost." Musical pieces that are indicated by type and number, such as Barber's "Cello Sonata Op. 6," do not get italicized, but long ones with names such as Handel's "The Messiah" do. Also ships, trains and rockets often are italicized.
Emphasis
In personal or creative writing, especially when writing dialogue, italics are used to indicate emphasis: "It's 'your' fault this happened!" Sometimes the emphasis is there for the sake of clarity, and sometimes it's simply to add expression. It's important not to use too many italics, or your writing will begin to sound melodramatic and even rather comical. Some writers also use italics to indicate a character's thoughts as opposed to her speech.
Words as Words and Others
Use italics for words you want to refer to as words, such as "'The' is the most common word in the English language" and for letters in the same manner. Also use italics for sounds you want to stand as sounds, such as "The bee went 'bzzz'" and foreign words that are not yet a normal part of English. For instance, you wouldn't italicize the word "petite," even though it originally was French, but you would italicize "bien."
Not Italicized
It may seem confusing to keep track of the different kinds of titles that are or are not italicized. A formal style guide such as the "MLA Handbook" or "Associated Press Stylebook" can give you detailed explanations; sometimes they vary on the same title. As a general rule, while larger artistic pieces, such as a book or TV series, are italicized, small sections within them, such as individual chapters or episodes, are not. Exceptions to this are sacred texts, such as the Bible or the Koran. Do not italicize punctuation marks or the word "the" at the beginning unless it is a part of the title itself. Car models and brands are not italicized; the rule about foreign words is open to interpretation.