CFormatter is one of Yii's core components and used to format values displayed in widgets like CGridView or CDetailView. All classic formats like numbers, text, urls or datetime are supported but you may have to use a formatting rule that isn't built in. Let's say we want to display arrays of strings (like tags associated with a product model). When using CDetailView we would usually make use of PHPs implode function to create a simple string value of all values in the $product->tags array.
More or less I use auto-complete field for a form. CJuiAutoComplete is a yii widget for this purposes.
The common use case is to make an ajax request and retrieve a list from the database. So in this case you have to write a controller action to return you a json encoded list.
What if I had a generic action to do this work for me?
Sometimes it would be nice if we could run yiic from an action, for example a migrate database button in our admin panel.
Or because you are on a host which doesn't have shell access.
Or maybe you are not allowed run popen or exec.
Yii's by-default directory organization works well enough, but there are several steps one can take that improve the security and serviceability of the system, especially in the context of multiple Yii projects on the same machine (including multiple versions of the same project).