LINQ (Language Integrated Query) allows adding querying capabilities to .NET languages. It defines standard query operators and translation rules to query data like arrays, XML, databases. LINQ to XML represents XML as XElement objects that can be queried using LINQ. The System.Xml.Linq namespace contains classes like XDocument and XElement for constructing XML documents programmatically. XML can be loaded from files, traversed, inserted, deleted, and updated using LINQ to XML.