You are clearing all the junk on your hard disk. Shift+Del. Shift+Del. Shift+Del. Oops you just deleted that most important file. Now if you have google desktop search installed you may get back your data. Search for the file name, say "veryimportant.doc" and sort by relevence. Your file must be visble. Next to the description you will find a link called cache. If you click, you will be taken to a webpage which will contain the content of that file. Google keeps a cache of files on your desktop so that they may be easily searched. You are using that cache to get back your file.

How to get your website back without a backup.
Imagine this.You create a website. Burn the midnight lamp on all that CSS, HTML and what not. And just after you upload your website your server crashes. No backups. Are you doomed? Google to the rescue. This is based on the fact that google keeps a cache of all websites they crawl. And you can view that cache.

Suppose your site is xyz.net

In google search box type site:xyz.net Google will show all the pages of your site. But if you want to visit them, obviously you can't. Your webhost has just lost them. Google to rescue. Next to your website address, there is a link called cached. If you click this link you will be taken to a page which will contain text of your webpage. So if you have some time to waste and data to recover, You can get all your pages back. (Sans the css and images.)