How does a computer store webpages that you have visited so that upon examination of the harddrive at a later date, it is possible to re-visit those pages whether they are fragmented or complete and take screen shots of them?
In a cache folder. This folder is usually set to a specified size limit so old content may be overwritten if not manually wiped out. This way your hard drive won't fill up either.
In IE, you can see the folder's location under Tools -> Internet Options -> Browsing History -> Settings:
Firefox does things a little differently and the files are stored in your random profile directory with different file names. You can view the cache files as well as the folder location by navigating to about:cache from the address bar.