Here's what I've been told:
Windows 7 and lower: Defender does only Malware
Windows 8 and above: Defender does Antivirus as well
Server 2008 Standard and below: Defender does only Malware
Server 2008 R2 and above: Defender does Antivirus as well
Is this correct? If not, what is?
Viruses come under the category malware; malware is defined as any "software which is specifically designed to disrupt or damage a computer system". I believe that, for Win7 and lower, Defender only protects against spyware (another form of malware) and nothing else. Microsoft Security Essentials is the equivalent, but acts as an anti-virus.
I'm not sure about the Server software as I've never been acquainted with them.
Windows Defender is a client product and is not supported on any server OS. There was a time when it would run on a server but I believe recent versions will not install. Most other AV products designed for a client OS won't run either. The installer specifically tests for and will not install on a server OS. There are many server AV products but few are free.
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