LINQ to XML provides a really powerful way to efficiently query, filter, and shape/transform XML data. You can use it both against local XML content, as well as remote XML feeds. You can use it to easily transform XML data into .NET objects and collections that you can further manipulate and transfer across your application.
LINQ to XML uses the same core LINQ query syntax and concepts that LINQ to SQL, LINQ to Objects, LINQ to SharePoint, LINQ to Amazon, LINQ to NHibernate, etc. use when querying data.
One major open question: how do LINQ to XML and XQuery relate, over time?