See test/rename.spec.js for more examples and test/path-parsing.spec.js for hairy details.
dirname is the relative path from the base directory set by gulp.src to the filename.
gulp.src() uses glob-stream which sets the base to the parent of the first directory glob (*, **, , or extglob). dirname is the remaining directories or ./ if none. glob-stream versions >= 3.1.0 (used by gulp >= 3.2.2) accept a base option, which can be used to explicitly set the base.
gulp.dest() renames the directories between process.cwd() and dirname (i.e. the base relative to CWD). Use dirname to rename the directories matched by the glob or descendents of the base of option.
basename is the filename without the extension like path.basename(filename, path.extname(filename)).
extname is the file extension including the '.' like path.extname(filename).