What is LINQ ?

LINQ stands for Language Integrated Query,which was introduced with .Net framework 3.5.It is used to query data sources as strongly typed queries.It defines a set of Standard Query Operators that helps it to get data from different data sources, Query expressions, Lambda expressions and anonymous types.

There are few types of usage

LINQ to Object
LINQ to Objects allows to perform queries against arrays and in-memory data collections.
It is the IEnumerable<T> API for the Standard Query Operations.

LINQ to XML

Is the LINQ API for manipulating XML. Operations are available in System.Xml.Linq namespace.

LINQ to DataSet

Is the LINQ API for DataSets.

LINQ to SQL

Is the IQueryable<T> API that allows LINQ queries to work with Microsoft SQL Server database.
System.Data.Linq namespace will be required

LINQ to Entities

Is the alternate LINQ API used to query a database. It decouples the entity object model from the
physical database by creating a logical mapping between the database and objects.