LINQ Basics, Getting started with LINQ and LINQ Queries

LINQ is a set of extensions to the .NET Framework that encompass language-integrated
query, set, and transform operations. It extends C# and Visual Basic with native
language syntax for queries and provides class libraries to take advantage of these
capabilities.

After two decades, the industry has reached a stable point in the evolution of object-oriented
(OO) programming technologies. Programmers now take for granted features like classes,
objects, and methods. In looking at the current and next generation of technologies,
it has become apparent that the next big challenge in programming technology is
to reduce the complexity of accessing and integrating information that is not natively
defined using OO technology. The two most common sources of non-OO information are
relational databases and XML.

LINQ to SQL provides a runtime infrastructure for managing relational data as objects
without losing the ability to query. Your application is free to manipulate the
objects while LINQ to SQL stays in the background tracking your changes automatically.

All the basic information about LINQ that you like to know(msdn link)
101 LINQ Examples