[xsl] Newby XSLT symbol question

["Mark Wilson" <drmark@xxxxxxxxxxxxxxx>]

I am brand-new to XSLT and have encountered symbols associated with 
variables for which I cannot find a definition. I tried the archives, but 
could not figure out how to search for symbols. I am trying to learn from 
Jeni Tennison's XSLT 2.0. She says on page 236, "...you can refer to that 
variable's value within another XPath expression using the variable's name 
prefixed with a dollar sign ($)." OK, that's clear, but what is the '@' sign 
used for? For instance, at the top of page 237:
<xsl:if test="@rating > $minRating">
Are there other such symbols and where do I find them defined?
Thanks,
Mark