LINQ is a uniform programming model for any kind of data access. LINQ enables you to query and manipulate data independently of data sources. Below figure 'LINQ' shows how .NET language stands over LINQ programming model and works in a uniformed manner over any kind of data source.It’s like a query language which can query any data source and any transform. LINQ also provides full type safety and compile time checking.
LINQ can serve as a good entity for middle tier. So it will sit in between the UI and data access layer.