clipped from msdn.microsoft.com

Querying data in server code has become significantly easier and more powerful in SharePoint Foundation 2010.

When your code needs to query list data, it is no longer necessary to compose a Collaborative Application Markup Language (CAML) query. SharePoint Foundation 2010 includes a LINQ to SharePoint provider to enable your code to query SharePoint lists from server code by using LINQ syntax. The provider translates your LINQ queries into CAML queries for submission to the content database. Because SharePoint Foundation 2010 also adds a new Join element to the Query Schema, you can have both implicit and explicit joins in your LINQ queries by using the join keyword. (See "List Joins Supported in CAML Queries" below.) You can also write to the lists by using the provider. As always with a LINQ provider, you must have classes that provide an object-oriented interface over the relational content databases.