> using XSLT 1.0 and Saxon 6.5.5 I'm wondering whats the actual 
> difference between
> 
> <xsl:key name="child-key" match="child" use="parent::*"/>

That indexes an element on the string-value of its parent node, which
doesn't sound a very useful thing to do.
> 
> <xsl:template match="parent" >
>         <xsl:variable name="childs" select="key('child-key', 
> parent::*)"/>
>         ...
> </xsl:template>
> 
> and
> 
> <xsl:key name="child-key" match="child" 
> use="generate-id(parent::*)" />
> 

This effectively indexes an element on the identity of its parent node -
which doesn't sound very useful either.

Michael Kay
http://www.saxonica.com/