Difference between 'where' and 'if' in an XSLT script

I am new to XML/XSLT, and I'm a bit confused about the difference between the <xsl:if test="x"> and adding a where="x" at the end of a statement.

Below is some example data and two XSLT versions of code. I tried running it both ways here: https://www.w3schools.com/xml/tryxslt.asp?xmlfile=cdcatalog&xsltfile=cdcatalog_ex1 but nothing appears, so I may be doing something wrong. Is anyone able to clarify this for me?

 <?xml version="1.0"?>  
 <Tests xmlns="http://www.adatum.com">  
   <Test TestId="0001" TestType="CMD">  
     <Name>Convert number to string</Name>  
     <CommandLine>Examp1.EXE</CommandLine>  
     <Input>1</Input>  
     <Output>One</Output>  
   </Test>  
   <Test TestId="0002" TestType="CMD">  
     <Name>Find succeeding characters</Name>  
     <CommandLine>Examp2.EXE</CommandLine>  
     <Input>abc</Input>  
     <Output>def</Output>  
   </Test>  
   <Test TestId="0003" TestType="GUI">  
     <Name>Convert multiple numbers to strings</Name>  
     <CommandLine>Examp2.EXE /Verbose</CommandLine>  
     <Input>123</Input>  
     <Output>One Two Three</Output>  
   </Test>  
   <Test TestId="0004" TestType="GUI">  
     <Name>Find correlated key</Name>  
     <CommandLine>Examp3.EXE</CommandLine>  
     <Input>a1</Input>  
     <Output>b1</Output>  
   </Test>  
   <Test TestId="0005" TestType="GUI">  
     <Name>Count characters</Name>  
     <CommandLine>FinalExamp.EXE</CommandLine>  
     <Input>This is a test</Input>  
     <Output>14</Output>  
   </Test> 
 </Tests>

Using where my XSLT is:

 <?xml version="1.0" encoding="UTF-8"?>
 <xsl:output method="text" version="1.0" encoding="UTF-8" indent="no"/>
 <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
 <xsl:template match="/">
   <xsl:for-each select="Tests/Test" where="@TestType='CMD'">
     <xsl:value-of select="current()">
   </xsl:for-each>
 </xsl:template>
 </xsl:stylesheet

Code using the if statemtent

 <?xml version="1.0" encoding="UTF-8"?>
 <xsl:output method="text" version="1.0" encoding="UTF-8" indent="no"/>
 <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
 <xsl:template match="/">
   <xsl:for-each select="Tests/Test">
     <xsl:if test="@TestType='CMD'">
       <xsl:value-of select="current()">
     </xsl:if>
   </xsl:for-each>
 </xsl:template>
 </xsl:stylesheet