A namespace is used to organize classes into a hierarchy, often to group types with related functionalities.
In PowerShell, the System namespace is implicit.
The System.AppDomain type was used previously; this command, used when introducing assemblies, can be shortened:
[AppDomain]::CurrentDomain.GetAssemblies()
The same applies to types with longer names, such as System.Management.Automation.PowerShell:
[Management.Automation.PowerShell].Assembly