As you and other people begin to create content for your site, things might begin to get organizationally challenged. Maybe you have lots of different pages of content, but no categories to organize them. It would take days to go back and tag everything that has been created. Fortunately, for Drupalers around the globe, there is a automated solution called OpenCalais (AKA Calais) that will do the heavy lifting for you.
• OpenCalais reads the content of your nodes, as you create them, or in bulk.
• OpenCalais uses some amazing analysis to create tags for your content