What is the difference between an AD cmdlets and a powershell command?
Is AD commandlet(for example : Get-ADUser) a powershell command?
Yes Get-ADUser is a PowerShell cmdlet, it is part of the Active Directory module. The difference between a cmdlet from a module and a default built-in cmdlet like get-help is that you must import extra modules to get the extra cmdlets. See Importing Modules for more.