XPath expressions

Processing language must incorporate a declarative query
syntax for selecting the data that needs to be processed. In SQL, that's the
SELECT statement. In XSLT, the equivalent is the XPath expression.

--Book from Micheal Kay

Means we have to pass single quotes within double quotes like this.
Is this method called the XPath expression?

<xsl:template name="str:tokenize">
    <xsl:param name="args" select="'$args'" />
    <xsl:param name="delimiters" select="' '" />

It will be surely there in later part of the book.But can anyone tell
me now which are exactly XPath expressions?

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Thanks and Regards,
Himanshu Padmanabhi