The Historical Answer
With Windows NT, prior to the advent of Active Directory, there was one Primary Domain Controller (DC) per domain, and every other DC was a Backup.
The Modern Answer
When Active Directory was introduced with Windows 2000, domain controllers became fully multi-master. There is no primary domain controller. However, there are two caveats that may confuse you.
FSMO Operations Masters
Though Active Directory is multi-master, certain operations must take place in a way that ensure consistency across the domain controllers. For this reason, one DC will be designated as the Operations Master for each role. This is not the same thing as a Primary Domain Controller, because there are currently 5 master roles, and a different DC could hold each role. To view the current Operations Masters, run the following at a Windows command prompt:
NetDOM /query FSMO
In Powershell, you have to run two commands:
Get-ADDomain | Select-Object InfrastructureMaster, RIDMaster, PDCEmulator
Get-ADForest | Select-Object DomainNamingMaster, SchemaMaster
Emulated Primary Domain Controller
One reason people still think there is a primary domain controller is that Active Directory emulates a primary DC (PDC) for backwards compatibility with Windows NT. Here are two ways to see which DC is “primary”. Note that the designation of “primary” is meaningless for a “modern” domain.
The commands in the previous section show which DC is the emulated PDC. Here are two more ways to show the PDC:
At a Windows command prompt:
Get-ADDomainController -Discover -Service PrimaryDC
Reference: Which domain controller is primary?