I'm writing a bio for a friend, and I mention many hotels and restaurants, some of which are foreign. Should I use italics?
I would say no. Here is a quick guide on the correct usage of italics. To summarize: Don't use it for the proper noun. They example they give for a restaurant: if you write about a certain dish you ate that might not be commonly known, italicize it, but do not italicize the restaurant name.
There is the consideration that a restaurant name might be confused for a geographical location in some cases; you could use quotation marks around the name to avoid confusion.
In which cases is a word, or a group of words written in italics?
Is italics used in specific contexts, or it is quite normal to write words in italics?
The Wikipedia page on Italic type gives a pretty good overview, along with some examples.
(See also this About.com page, though it says very similar things to the Wiki page.)
Hope that helps.
Off the top of my head, italics are used for:
In the first and second case, you could just as well enclose the word(s) in quotes (without using italics). The third one seems to be set in stone.
The other answers have many good examples of when to use italics, but I wanted to emphasize that on this site in particular I most frequently use italics to clarify the use–mention distinction—that is, to mention a word rather than use it. Of course, in the previous sentence I used italics to emphasize and not for the use–mention distinction. Quotation marks are also frequently used to clarify the use–mention distinction, but I tend to reserve them for mentioning multi-word phrases and use italics for single words.