Microsoft Active Directory (AD) service is usually what a DC is running. AD back bone is LDAP, to put it simply. Basically a LDAP server could be called a 'domain controller' as it serves the same function as a DC.
Just to correct your assessment. You should have said "Macs and Linux machines don't have to be plugged in to a DC". This would be correct, and nether do Windows machines. Depending on the size of the environment these machines do need be plugged in to a DC.
If you have a only a handful of machines, say less than 20, then its fine to go along with out a DC. But even in this environment its better to have one. In a medium or large organization having a DC forest set up is essential. I would almost say required.
Since you do not work in IT this would be a good thing. Please do not move into this field, less people will hate you in life and I doubt you would be happy there.