.Pages = Single page = empty slice
.Pages = List page = Regular pages under that list page representing the homepage, section, taxonomy term (/tags) or taxonomy (/tags/foo) page – Subset of .Site.Pages or .Site.RegularPages, depending on context
.Site.Pages = any page = ALL pages of the site: content, sections, taxonomies, etc. – Superset of everything!
.Site.Regular.Pages = any page = Only regular (content) pages – Subset of .Site.Pages
.Data.Pages = All the Pages for the given Node (sections, taxonomies). This will be a subset of the above for all other nodes than the home page: On that Node I believe these slices are the same
In the home context (index.html), .Pages is the same as .Site.RegularPages.
.Data.Singular = The singular name of the taxonomy (e.g., tags => tag)
.Data.Plural = The plural name of the taxonomy (e.g., tags => tags)
.Data.Pages = The list of pages in the taxonomy
.Data.Terms = The taxonomy itself
.Data.Terms.Alphabetical = The taxonomy terms alphabetized
.Data.Terms.ByCount = The Terms ordered by popularity
Note that .Data.Terms.Alphabetical and .Data.Terms.ByCount can also be reversed:
.Data.Terms.Alphabetical.Reverse.Data.Terms.ByCount.Reverse