Although you will still see the double quotation marks used to quote direct speech it is more and more common to punctuate speech and direct quotations with single quotation marks with the double quotation marks reserved for quoting ‘speech within speech’.
‘I haven’t spoken to Peter for months,’ Dianne said. ‘The last time I spoke to him he said, “I’m going to Bahrain and won’t be back for about three years”, I’ve heard nothing since then’.
Notice that the comma is placed within the quotation marks and that double quotation marks are only used when the quoted speaker is quoting someone else directly.
Other uses of the quotation mark
You will also have seen the single quotation marks used to mark out idiomatic expressions such as:
I’ve always thought that he was very annoying, a bit of a ‘pain in the neck.’
They are also used outside of speech when quoting the title of a journal article:
‘The Migration Flight of the Lesser Tweazle‘, by Jeremey Adams, in The Bird Spotter Magazine, July 2009.
Note that there are a number of ways of organising such things as bibliographies which set out standard formats. Most organisations and academic institutions will prefer one of these or have their own format published in a ‘style guide’.