To access popular .NET types faster, PowerShell maintains a list of shortcuts called "type accelerators". That's why you can use the term "XML" for XML data instead of having to always write "System.Xml.XmlDocument":
PS> [xml].FullName System.Xml.XmlDocument
The list of type accelerators can be extended, so if you have additional .NET types that you use often, you can add them to that list.
This line adds a new type accelerator called "Path" that refers to the type "System.IO.Path":
You can now access all the highly useful static methods in that type very easily:
PS> [Path]::GetFileNameWithoutExtension('test.txt') test
Note that type accelerators are just for your convenience. You can always use the full type name instead, too:
PS> [System.IO.Path]::GetFileNameWithoutExtension('test.txt') test