A collection of patterns to ensure a domain layer independent of underlying technology.
Back in the old days of client/server life was fairly simple. You could find the a way to get the data you needed from the database, usually using ODBC, and splash that data to a form, usually a Windows form. You could build applications in a jiffy. Didn't we all love PowerBuilder, SQL:Windows and Visual Basic?
Then life got a little more complicated at the time the customer wanted some validation in the application. Let's say hew wanted to make sure that the end date of a contract was either blank or later in time than the starting date. Here's where the discussion started. The developers that previously worked with Clipper or DBase would insist to put this validation in the database. And they did. Nice and safe. The younger developers, including me, however would mention that if validation was delayed until the data reached the database, it was fairly late. Couldn't we just build the validation into the form? Well, yes we could. And we did.
The nasty thing about this architecture is that both arguments hold. Yes, validations in the database is fairly safe. And yes, validating until the data is in the database is fairly late. So now we keep two versions of our little validation rule. One in the database, and one in our form. It's the same rule. For now.