The MSDN Site: http://msdn.microsoft.com/en-us/vbasic/aa904594.aspx
LINQ (Language Integrated Query) was delivered in .NET 3.5 and Visual Studio 2008. It addreses O/R mapping issues by making query operations like SQL statements part of the programming language. It also offers built-in support for querying in-memory collections like arrays or lists, XML, DataSets, and relational databases.It also allows you to develop queries in your native development language (with IntelliSense).
LINQ to XML: It is recommended that developers switch over from coding against the XmlDocument DOM to LINQ's XDocument
LINQ to SQL: Deprecated... use LINQ to Entities
LINQ to Entities: Entity Framework Learning Guide