Cached is the old version of your page that Google has. Supplemental is sometimes referred to as "Google hell". It usually means that your page has little or no inbound links so Googlebot sees it as non important. But it doesn't mean it can't rank.
the cache is a snapshot of your website when google indexed it. the supplemental results are part of google's auxiliary index, often referred to as "google hell". in other words, the supplemental pages are the red-headed step children of the serp's. pages usually get placed into the supplemental results when they: A. do not have any pr of their own or B. are very similar, if not identical to other pages in your site (lookup duplicate content).
Supplemental results can certainly still rank - but will not generally if there's a similar page not in the index to show instead. I've found that you still get some traffic - but it's generally only on long-tail phrases. Dylan
Supplemental results are the page(s) of your site that may rank for your main keyword as well.. Cache is a version of your site downloaded into google's database which is viewed directly off google.
google does not show "supplemental results" any more.......... but still if you want to know Cache is the latest snap of your site in google's index Supplemental pages are those pages that are in google's secondary index and it gives less importance to those pages