I may be wrong, as it's been some time since I worked with them, but I think XPath is both its own standard, and a part of XSL at the same time. A lot of XSLT deals with selecting nodes from the source and it happens with XPath expressions.
W3C standards usually depend on and leverage other standards, so XPath is its own standard, which is used by XSLT (and XQuery, and possibly a few other things).
You can't use XSLT without XPath, but you can use XPath on its own.
