Onion Architecture is a specific type of solution architecture that enforce true loose coupling. This video shows that the User Interface layer has no reference to any infrastructure/DAL, just the Core business logic. Even the business logic (Core) has no access to any infrastructure concerns. Rather than build on top of the database, it externalizes it. It defines what it needs using its own interfaces, and they are implemented as far outward as possible in Infrastructure. You can then bind them together using an IoC container to bring the application to life.
Video Producer: http://chicagoalt.net/
More information on the Onion Architecture