This evening I spent some time digging into more about Power Query. I found that there is a language of Power Query, called M, and a library that provides values available to expressions. Pointers to the language and library specs can be found on this web page.
Note that Power Query is available in 2 flavors – a released version and a preview that goes along with Power BI. There are different specs for each.
It appears that Power Query is evolving and releasing at a relatively rapid rate. It will pay to keep up with the latest releases as they are addressing feedback to the community with every release.