They are called scare quotes. Here it the Wikipedia entry:
In journalism, scare quotes are quotation marks used for any other purpose than to identify a direct quotation, such as for emphasis or irony.
Although the term's connotation is not necessarily negative, an author who uses the term scare quotes to describe them generally does so disapprovingly. The effect of using scare quotes is similar to inserting so-called to modify a word: in both cases, the object is to express disagreement with the word, but only by calling attention to the word without actually explaining why the author disagrees with it. For this reason, scare quotes are sometimes called sneer quotes." In some cases, the scare quotes may be omitted without loss.
In spite of their pejorative label, such quotes may be used legitimately. An author who uses quotation marks in such a manner may do so in order to disclaim responsibility for the words, or to emphasize that a specialized, narrowed or historical sense of the quoted material is being suggested.
Scare quotes are also sometimes called orphan quotes. The spoken equivalent of scare quotes are known as air quotes or finger quotes.