This is a library used by some of the other LINQ samples.
The NorthwindMapping.DBML file in this project generates the custom CLR classes used to access the underlying SQL Server database. By changing the properties in the designer for NorthwindMapping.DBML, you can modify the classes that are generated and ensure that you have the exact datatypes and object relationships that you want. You can see the resulting classes by looking at the generated code-behind file, NorthwindMapping.designer.cs.