I guess that by the content of a tag you mean the stuff between the tags. Correctly speaking, given <a>xyz</a> there are two tags, whose content is "a" and "/a" respectively. It's better to talk about element nodes and their children - the input to an XSLT transformation is a tree of nodes, not a string containing angle brackets. Using the right terminology will help you find the right information.
If you want to copy the children of an element node, and that element is the current context node, use <xsl:copy-of select="child::node()"/>
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference