A model is an instance of CModel or its child class. Models are used to keep data and their relevant business rules.
A model represents a single data object. It could be a row in a database table or a form of user inputs. Each field of the data object is represented as an attribute of the model. The attribute has a label and can be validated against a set of rules.
Yii implements two kinds of models: form model and active record. They both extend from the same base class CModel.
A form model is an instance of CFormModel. Form model is used to keep data collected from user inputs. Such data are often collected, used and then discarded. For example, on a login page, we can use a form model to represent the username and password information that are provided by an end user. For more details, please refer to Working with Form
Active Record (AR) is a design pattern used to abstract database access in an object-oriented fashion. Each AR object is an instance of CActiveRecord or its child class, representing a single row in a database table. The fields in the row are represented as properties of the AR object. Details about AR can be found in Active Record.