Bulletproof hosting is hosting that withstands high amount of web requests in short time interval. This is also known as a Slashdot effect – your web site or page is linked from a very popular portal and thousands of visitors go to your web site causing high amount of web requests. This usually slows down your web site and usually your site is unreachable – server doesn’t have enough resources to serve all requests.
This is very load balancing and clustering comes in – you share the load between multiple hardware boxes, thus your web site is accessible even it’s accessed by thousands of simultaneous web users.
Load is usually balanced in the front-end, thus the backend database is still one or it could be a cluster of databases.