By your "simple" question I guess you just checked some Yii code here and there without reading any tutorial… I would strongly suggest you to read the Definitive Guide to Yii - http://www.yiiframework.com/doc/guide/
It will give you all the starting information you need to understand the Yii basics.
Quite right. I made the mistake when I first started with Yii of using CFormModel for a form and CActiveRecord to persist the data. It just about doubles the work load and makes maintenance more difficult.
As mdomba says, use CActiveRecord throughout if you’re persisting data from the form to a database. Use CFormModel for things like login forms where you’re just confirming data that the user has entered.