ECO, Enterprise Core Objects, is a model driven framework. In essence, it allows you to specify your application using a UML class model. This model is then transformed to source code, decorated with enough information to re-create the model information at run time.
The framework uses the model information (as contained in and re-created from the source code) to drive persistence, presentation, maintain the technical integrity of the business objects, manage bi-directional relations, derived associations and attributes, maintain constraints, offer services like undo/redo, object versioning and quite a bit more.
The value added proposition for you is that you can design your application on a 'higher level', without worrying about implementation details. Use the model not only for communication of ideas, but also as a part of your application. You can code your business logic on the same abstraction level as you design it. Typically this will allow you to produce more advanced applications faster, using less code. Less code means fewer bugs and simplified maintenance.
What is ECO?
This question has been asked so many times. Jesper (one of the developers) recently explained it like this...