An excerpt from http://www.sitepoint.com/high-availability-wordpress-with-hyperdb/, by James George
One of the reasons why I have always been so enamored with WordPress is because of the limitless possibilities it has. It seems like if you know PHP and how to leverage data and control it, you can do almost anything. A WordPress database can hold just about any type of data, which has made it even more useful with the added capability of custom post types and custom fields. You can save all sorts of data to a WordPress database.
The problem arises when you collect too much data, which can bog down your servers. Imagine huge websites with thousands of members, storing all sorts of information. This could be anything from member profile data, such as names, locations, and bios, to product information for a business website. One of the challenges is the limitation of using a single database and a single server to handle all of this. What happens when your lone server fails? The whole site and it’s functionality comes tumbling down, until the server is restored to working order.
Along comes HyperDB, a plugin for spreading your site’s load across several servers and databases.
HyperDB has its advantages, which we will discuss as we continue our exploration. Why would you want to use HyperDB for your WordPress install? Here are just a few features.