Widgets Or Partial Views?

So I am fairly new to Yii and reading various books and guides has led me to a question. If I want to reuse some code, whether it is complex or simple, that I can do so using widgets or partial views.

I am wondering if I have missed something, should these be used for different features or is there an overlap in functionality, why have both?

Ok I am not Yii expert dont know what the exact diff is but have few years of experience I make use widgets, inside the widgets I do use render partial if i need it in more than one widget somewhat similar to CGridList, one more thing renderPartial is used for ajax extensively.