XSL and FO

Good Morning experts,
i would like to ask you a question: i have an XSL file with <xsl:template>
and <fo:...> tags... I looked that some users made an FO file with FO tags
only. So my qustion is: what is the different between the files XSL and FO?
I tried to put html tags also with template and FO tags into XSL file, but
during the processing (to transform in PDF), FOP doesn't work... Why?
I hope you can help me.... Thank you...
Regards
     Gianni