Active Directory (AD (Active Directory)) is Microsoft's on-prem directory service. It holds user accounts, computer accounts, and groups, organized into Organizational Units (OUs).
A domain user can log into any joined PC (Personal Computer). A local user only exists on one machine. Most password-reset tickets are AD users — use Active Directory Users and Computers (ADUC (Active Directory Users and Computers)).
Groups grant permissions in bulk. Never assign permissions directly to a user — put the user in a group and grant the group.