What is the difference between Cached Pages and Indexed Pages?

Cached Pages

Google takes a snapshot of each page it examines and caches (stores) that version as a back-up. The cached version is what Google uses to judge if a page is a good match for your query.
Practically every search result includes a Cached link. Clicking on that link takes you to the Google cached version of that web page, instead of the current version of the page. This is useful if the original page is unavailable because of:

• Internet congestion
• A down, overloaded, or just slow website
• The owner’s recently removing the page from the Web

Sometimes you can access the cached version from a site that otherwise require registration or a subscription.

Note: Since Google’s servers are typically faster than many web servers, you can often access a page’s cached version faster than the page itself.

If Google returns a link to a page that appears to have little to do with your query, or if you can’t find the information you’re seeking on the current version of the page, take a look at the cached version.
Let’s search for pages on the Google help basic search operators.

And indexing is the process of storing URLs alone to the database. And, the indexes are not such data, but rather pointers to it.

By way of analogy, just as the index of a book tells you on which page(s) to find a particular subject, so too the Search Engine's indexes point to the place(s) in its database to find things.

For more information please visit Google Guide Page.