In Active Directory, groups are used to group user accounts and computer accounts into units that are then easier to manage. First, it is important to understand why groups and group memberships are relevant.
How to find a specific “Object of Interest” or even several objects that have certain properties? The answer to this is as logical as it is simple: you need to know and specify the right search filters.
In Active Directory, domains are used to emulate organizational structures; a domain is always an organizational unit with a unique name that contains, among other things, specific security policies and settings.