Google Cache is normally referred as the copies of the web pages cached by Google. Google crawls the web and takes snapshots of each page as a backup just in case the current page is not available. These pages then become part of Google's cache. These Google cached pages can be extremely useful if a site is temporary down, you can always access these page by visiting Google’s cached version.
Google web is usually updated in a few days. The actual time of the updates depends on the frequency the website updates itself.