When to use ViewModel in mvc3

In asp.net mvc, a controller can pass single object to the corresponding view. This object may be a simple object or a complex object. ViewModel is a complex object that may contain multiple entities or objects from different data models or data source. Basically ViewModel is a class that specify data model used in the strongly-typed view. It is used to pass data from controller to strongly-typed view.

Designing the model

public class UserLoginModel
{
[Required(ErrorMessage = "Please enter your username")]
[Display(Name = "User Name")]
[MaxLength(50)]
public string UserName { get; set; }
[Required(ErrorMessage = "Please enter your password")]
[Display(Name = "Password")]
[MaxLength(50)]
public string Password { get; set; }
}