What the processing Instruction mean.

Hi
 can anyone tell me what the processing instruction mean.
When we write xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
what actually it means does it look for a DTD on that site.
Similarly for xml we write 
<?xml version="1.0"?>
-----------------------------------------------------
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"> 
-----------------------------------------------------

Thanks in Advance
-Sanghamitra


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