Roman (that is upright type) is the standard text typeface, but for various reasons italic type (that is slanted to the right) is adopted. This is often to indicate some form of departure from normal text for the reader to interpret the word or words in a certain way. Some of the main uses of italic type, or italics, in British English follow.
Italic type is generally used for titles of:
• published books, except for the Bible, the Koran, and books within the Bible (notice the capitalization of these two important works);
• plays, films / movies, TV and radio series, for example West Side Story;
• long poems of book length or divided into books or cantos, such as Milton’s Paradise Lost;
• magazines and journals (‘The’ is usually omitted from bibliographical references of such titles, but retained as lower-case roman ‘the’ in ordinary text – two exceptions being The Times and The Economist);
• works of art such as paintings and sculptures, for example Constable’s The Hay Wain;
• major musical works such as operas, oratorios, and ballets, such as Coppélia (note that roman should be used for nicknames given by other people to works, such as in Beethoven’s ‘Pastoral’ Symphony);
• ships, aircraft, and vehicles, for example HMS Belfast (note HMS is always roman), the Concorde airliner, and The Spirit of St. Louis. (Note that italics are used for individual names but not for models such as Boeing 747 Jumbo Jet or a Ford Focus car.)
• foreign phrases not yet accepted (naturalized) as part of the English language, and many Latin terms;
• to emphasize certain words or phrases as a distinction;
• stage directions in plays;
• cross-references in such as see or see also in an index;
• other directions to the reader, such as opposite;
• dictionaries for part of speech markers, foreign words in etymologies, and usage labels;
• names of parties in legal cases;
• biological nomenclature, such as genera and species;
• mathematical variables;
• some medical terminology, such as bis die for twice a day;
• for emphasis and highlighting, though should be used sparingly;
• lists, such as in (a), (b), and (c).
Note that italic punctuation should only be used within an italic phrase and not before or after it.
If you are writing for a specific publisher you should always check their house style for the use of italics in case they have specific, non-standard requirements.
When foreign words have been sufficiently assimilated into the English language over time and usage they tend to change from italic to roman, sometimes retaining their accents as in crèche, or sometimes losing them as cafe and elite. In modern English, the use of italics for foreign words occurs less than it used to and can pass from italics into roman quite quickly.