XSLT Stylesheets are XML documents; namespaces (http://www.w3.org/TR/REC-xml-names) are used to identify semantically significant elements.
Most stylesheets are stand-alone documents rooted at <xsl:stylesheet> or <xsl:transform>. It is possible to have "single template" stylesheet/documents.
<xsl:stylesheet> and <xsl:transform> are completely synonymous.
Note that it is the mapping from namespace abbreviation to URI that is important, not the literal namespace abbreviation "xsl:" that is used most commonly.