Under Windows 10 and Windows 8 Windows Defender is an antivirus and antimalware tool with realtime scanning.  It is getting antiransomware and several other capabilities integrated into it with the fall update this year.

Under Windows 7 Windows Defender was an antimalware scanner and Microsoft Security Essentials was the antivirus.  Both could be used together.

Microsoft Security Essentials is unnecessary under any version of Windows later than Windows 7.

--
Brian  - Windows 10 Home, 64-Bit, Version 1703, Build 15063  (dot level on request - it changes too often to keep in signature)

 
If you don't like someone, the way he holds his spoon will make you furious; if you do like him, he can turn his plate over into your lap and you won't mind.     ~ Irving Becker