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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'.
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.
You will also have seen the single quotation marks used to mark out idiomatic expressions such as -
They are also used outside of speech when quoting the title of a journal article:
(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'.)