LINQ stands for Language Integrated Query and it is a set of .Net framework extensions that allow us to query various data sources using a .Net language. There are a couple of different flavors of LINQ:
a) LINQ to SQL: This is for applications that use objects mapped to the database objects.
b) LINQ to Entities: This is for applications that need more flexibility in mapping objects to a RDBMS supported by the ADO.Net data providers.
c) LINQ to XML: This provides an in memory XML API.
d) LINQ to object: This allows us to do queries against the in memory objects.
e) LINQ to Sharepoint: This allows us to query MS Sharepoint lists.
f) LINQ to DataSet: This allows us to query DataSets.
You can get more information on LINQ using these resources: