The Digg Protector will determine if a visitor is from Digg, and if the visitor is indeed from Digg, the plugin will serve them a remotely-hosted version of the image. Otherwise, the plugin will serve the locally-hosted (on that server) image.

To save your SQL server resources, WP Cache works by caching Worpress pages and storing them in a static file for serving future requests directly from the file rather than loading and compiling the whole PHP code and then building the page from the database. It is a very useful plugin if you have a high traffic site or low performance server.