The System.Xml.Linq namespace was added with .NET 3.5. This is known as LINQ to XML. Language Integrated Query (LINQ) is used to describe a query in the same language as the rest of a program. Therefore, interacting with a complex XML document does not require the use of XPath queries.
PowerShell can make use of System.Xml.Linq once the required assembly has been added:
Add-Type -AssemblyName System.Xml.Linq
This can also be phrased as:
using assembly System.Xml.Linq
As a newer interface, System.Xml.Linq tends to be more consistent. The same syntax is used to create a document from scratch that is used to add elements and so on.