CSS3 Syntax : or :: ?

Throughout the new CSS3 documents I've noticed that both

::first-letter
and
:first-letter

are used in several, several examples. Which is the proper syntax? The latter comes from CSS2's pseudo-class syntax. Are they interchangable, both to mean pseudo-class? If so, why the introduction of :: ?

-Chris "SoopahMan" Moschini
http://hiveminds.info/
http://soopahman.com/

(ignore attachment)

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