I am curious about file extensions for style sheets.  I gather both xsl
and xslt are used, but I cannot tell what the difference between them
is.  Are there any rules or guidelines for this?

File extensions are
1. for the convenience of people, getting an idea of the content of
   a file by looking at the file name
2. for tools which associate behaviour with file extensions.

There are no further rules or guidelines beyond this to my knowledge.
Many tools related to XSL have preconfigured or even hardwired
behaviour associated with the xsl extension. I'm not aware of tools
which use xslt by default, but I use this often if I want to override
associations of certain tools while still being able to use the
default behaviour and keep hinting to XSLT content for people.