While you are dealing with DTO (Data transfer objects or data access objects) and VO (value objects) or view objects you will need to transform one into another. Especially if you are working with a multi layer system, and while using NHibernate, or any ORM, mapping these objects causes very ugly code.
AutoMapper provides mapping between objects and ease the development. Very similar to Fluent NHinerbate mapping.
You take one object map to another object. Instead of wiring the objects manually, you configure once and you are done. really sweet.
Automapper provides loose couple, mapping of layers are lot of work. Automapper makes translation of objects really easy.