namespace declarations

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="test.xsl"?>

Can someone explain or recommend some easy to read info on namespaces?
In my .xsl file I've been using
<?xml:stylesheet ... instead of <?xml-stylesheet...

Does it matter which one?
And can I combine the two lines
(<?xml version... and <?xml-stylesheet..)
together?

Confused,
-alex
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