XML : What does key does in select attribute?

I'm trying to understand to listing 18 of this page example. The listing looks like this:

  <?xml version="1.0" encoding="utf-8"?>  <xsl:stylesheet    xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/"    xmlns:xsl="http://www.w3.org/1999/XSL/Transform"    version="1.0">      <xsl:import href="wsdl-util.xsl"/>    <xsl:strip-space elements="*"/>    <xsl:output method="text"/>        <!-- just process the services -->    <xsl:template match="wsdl:definitions">      <xsl:apply-templates select="wsdl:service"/>    </xsl:template>        <!-- dump out some information for each service -->    <xsl:template match="wsdl:service">      <xsl:text>service: </xsl:text>      <xsl:value-of select="@name"/>      <xsl:text>  </xsl:text>        <xsl:apply-templates select="wsdl:port"/>    </xsl:template>        <!-- follow the linkages from the wsdl:port through the         wsdl:binding, back to the wsdl:portType and its          wsdl:operations -->    <xsl:template match="wsdl:port">        <!-- key for looking up the wsdl:binding -->      <xsl:variable name="binding-key">        <xsl:apply-templates select="." mode="binding-key"/>      </xsl:variable>        <xsl:for-each select="key('binding',$binding-key)">        <!-- key for looking up the wsdl:portType -->        <xsl:variable name="porttype-key">          <xsl:apply-templates select="." mode="porttype-key"/>        </xsl:variable>          <!-- process each operation in the wsdl:portType -->        <xsl:for-each select="key('porttype',$porttype-key)/wsdl:operation">          <xsl:apply-templates select="."/>        </xsl:for-each>      </xsl:for-each>    </xsl:template>        <!-- generate a Java-like method signature for each operation -->    <xsl:template match="wsdl:portType/wsdl:operation">        <xsl:text>  </xsl:text>        <!-- return type -->      <xsl:for-each select="wsdl:output">          <!-- get the correct value for the message name key -->        <xsl:variable name="output-message-key">          <xsl:apply-templates select="." mode="message-key"/>        </xsl:variable>          <!-- output the type of the first part of the message -->        <xsl:for-each          select="key('message',$output-message-key)/wsdl:part[1]">          <xsl:value-of select="@type"/>        </xsl:for-each>        </xsl:for-each>        <xsl:if test="not(wsdl:output)">        <xsl:text>void</xsl:text>      </xsl:if>          <!-- method name -->      <xsl:text> </xsl:text>      <xsl:value-of select="@name"/>          <!-- arguments -->      <xsl:text>(</xsl:text>      <xsl:for-each select="wsdl:input">          <!-- get the correct value for the message name key -->        <xsl:variable name="input-message-key">          <xsl:apply-templates select="." mode="message-key"/>        </xsl:variable>          <!-- output the type of the type and name of each part of             the message -->        <xsl:for-each select="key('message',$input-message-key)/wsdl:part">          <xsl:value-of select="@type"/>          <xsl:text> </xsl:text>          <xsl:value-of select="@name"/>          <xsl:if test="not(last())">            <xsl:text>, </xsl:text>          </xsl:if>        </xsl:for-each>        </xsl:for-each>      <xsl:text>)</xsl:text>          <!-- exceptions -->      <xsl:if test="wsdl:fault">        <xsl:text> throws </xsl:text>        <xsl:for-each select="wsdl:fault">            <!-- get the correct value for the message name key -->          <xsl:variable name="fault-message-key">            <xsl:apply-templates select="." mode="message-key"/>          </xsl:variable>            <xsl:for-each select="key('message',$fault-message-key)">            <xsl:value-of select="@name"/>          </xsl:for-each>            <xsl:if test="not(last())">            <xsl:text>, </xsl:text>          </xsl:if>          </xsl:for-each>      </xsl:if>        <xsl:text>;  </xsl:text>      </xsl:template>    </xsl:stylesheet>    

out of which they have written the code like this:

    <xsl:template match="wsdl:port">        <!-- key for looking up the wsdl:binding -->      <xsl:variable name="binding-key">        <xsl:apply-templates select="." mode="binding-key"/>      </xsl:variable>        <xsl:for-each select="key('binding',$binding-key)">        <!-- key for looking up the wsdl:portType -->        <xsl:variable name="porttype-key">          <xsl:apply-templates select="." mode="porttype-key"/>        </xsl:variable>          <!-- process each operation in the wsdl:portType -->        <xsl:for-each select="key('porttype',$porttype-key)/wsdl:operation">          <xsl:apply-templates select="."/>        </xsl:for-each>      </xsl:for-each>    </xsl:template>    

I could able to sense that the variable binding-key will be equal to the binding value of the WSDL that is processed, which might be like this ( for the given example ):

  binding-key : http://tempuri.org/test binding    

with that in place, I have few questions:

1. What does key() does with the $binding-key whose value is http://tempuri.org/test binding?
2. And what does mode means in the apply-templates