Chris Hoffman (@chrishoffman) wrote a very useful article explaining how these updates are released and what they represent:
The article is well worth reading but very briefly
CU – These are the Cumulative updates now called “quality updates”, released every 6 months.
B Updates – These are the typical “Patch Tuesday” released in the second week of the month. Updates containing important updates and security fixes. Unlike Windows 7 these roll-up previously released security fixes.
C and D Updates – These are release in the third and fourth week of the month and are regarded as “optional”.
The interesting thing about these is that these don’t automatically install. They are initiated when you press “Check for updates”.
So what is the problem?
The issue is that by pressing “Check for updates”, you have opted into being a “seeker”, i.e. an early preview tester. You are inadvertently installing test updates that haven’t gone to the main userbase yet.
Please check out the article above for more information and supporting sources.