I am a pretty advanced PHP coder but I have little experience in commercial website development.
I would like to know if Yii is considered a rival to the likes of WordPress or if they are simply just different types of application? If they are different, how to they differ? I have seen blog posts suggesting Yii is superior to the likes of Drupal etc, so I assume there must at least be a massive crossover between Yii and the likes of Drupal, Wordpress etc?
Drupal, Wordpress, Joomla etc are mainly CMS systems. Yii is just application framework on top of which you can build applications (they may of course be CMS applications too). This means you should not expect advanced administration panel and base functionality - you have to write your own based on code generators, ready modules, etc. Yii will provide just components to achieve those functionalities (like in Zend Framework, Code Igniter and similar frameworks)
Yii is highly object oriented so it rather far from what you have in Drupal, more like Joomla… but if you are familiar with OOP - you should give Yii a chance.