cache:-look of your site index:- copy of your site(snap short of your site take in the google database)
index means that when you start submit your web page in search engine then page is cached then is called index, this for say only one time, but cached every take by search engine for website update.
cache = indexed version of your site in the past. If you change your web to new page, you still able to see the old version of your site using cached version.
From cache data you can know that when google spider came on your webpage last time and visit your site. Google's cache is helpful because from that you should know that how often visit your site by spider. You can see the Google cache of your website/web pages like In the Google search result showing the URL that you need to view, click the "Cache" link. Indexing means whatever you can see in search result. If you search web design keyword related to your website and search engine show that particular page related to that keyword in search engine then you can say that google has been index your web page in it. There is no any specific relation between them but first cache is need after that you can see it in indexing results.
Google cache means Google is aware of your website or web page while Google index means Google have one copy of your website or web page in its database.