At the moment, there are 90 question tagged as macros, i.e. dealing with matters of "macro programming" (as in section R of the UK TeX FAQ). The site also features 37 questions tagged as newcommand, covering the do's and dont's of creating new commands (and modifying existing ones). But wait ... this is macro programming, isn't it?
Having tried to come up with a sensible distinction between the two "tag areas", I do believe there is none. The newcommand tag probably was created because
\newcommand is the first thing a nascent macro programmer will encounter, although macros is the general term. In accordance with that, the "harder" programming questions (the ones that e.g. Joseph Wright and Will Robertson excel in answering) tend to be tagged with macros. (That is a distinction, but not a sensible one -- we don't feature tags like tikz-pgf-easy and tikz-pgf-hard.)
EDIT: I created a tag synonym suggestion.