Your WordPress database stores all of your website content. This includes blog posts, pages, comments, and custom post types such as links, form entries, and portfolio items. It also stores website settings, theme settings, and plugin settings. If you update your website regularly, your database will grow larger over time. A large database can greatly affect the performance of your website as it takes longer for your server to retrieve information from database tables. This is why database optimization is so important. Share on Facebook Tweet it Share on Google+ Sign up for more WordPress wisdom
Source: Optimizing Your WordPress Database – A Complete Guide – WPMU DEV