Difference between these NameSpaces...

hello everybody:
	I'm a new guy who just begin to use XML.
and When I use the XSL to Transform XML ,I encounter some matter.
that is,when I define the		 xmlns:xsl= ...
usually there are several  that I can use .
http://www.w3.org/TR/WD-xsl
http://www.w3.org/1999/XSL/Transform


for the first one, I  found that in some book ,they often use the URL to
demonstrate. but I found It isn't support the Xpath.So I know It just
XSL,but I still wondered that the correct URL with the XSL1.0 that I had
found in W3c is :http://www.w3.org/TR/xsl/  but not :
http://www.w3.org/TR/WD-xsl


but I seeked in the w3c.org that the standard XSLT 1.0 is
:http://www.w3.org/TR/xslt   not  to
:http://www.w3.org/1999/XSL/Transform
I tried to exchange the URL  above,the the result is different. 
May be the puzzle is stupid, but I am really encountered!
Can any one help me?


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