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