The Nop.Data project contains a set of classes and functions for reading from and writing to a database or other data store. The Nop.Data library helps separate data-access logic from your business objects. NopCommerce uses the Entity Framework (EF) Code-First approach. Code-First allows a developer to define entities in the source code (all core entities are defined in the Nop.Core project), and then use EF to generate the database from the C# classes. That's why it's called Code-First. You can then query your objects using LINQ, which translates to SQL behind the scenes and is executed against the database. NopCommerce uses a fluent code API to fully customize the persistence mapping. You can find more about Code-First here or here.