Indexed means its a page that is within the search engine, a cached page means it is the cached webpage that google has last seen when they visited your website meaning it could be a different version then the live one... but as far as I know if there your page is cached it is more then likely already indexed in the search engine.

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As far as i understand.

Indexed = Google know about said page ( It may appear in search results )
Cached = Google simply have a "copy" of the page. So if your sites offline, People can still view Googles cached copy of it.

Hello........

An “indexed” page is a page that has been crawled by a search engine spider and filed away in that search engine’s index for later use, and a “cached” page is one that might actually show up in search results.

As per my knowledge:
Google Cache:- Google takes a snapshot of each page examined as it crawls the web and caches these as a back-up in case the original page is unavailable. If you click on the "Cached" link, you will see the web page as it looked when Google indexed it. The cached content is the content Google uses to judge whether this page is a relevant match for your query.

Google Index: Google creates the index and the database of documents that it accesses when processing a query that user enter in the search engine.

indexing - the act of classifying and providing an index in order to make items easier to retrieve.......

SEO is all about the cache. Crawl frequency determines how changes impact your site. Without an updated cache, there are no significant changes or rankings to report to search engines.