What Happened in the Kaseya VSA Incident?

Right before the start of the holiday weekend, news broke of yet another massive ransomware attack. The apparent Kaseya VSA supply chain attack is likely to affect thousands of businesses, with an apparent affiliate of the REvil Ransomware gang launching what seems to be  a particularly pernicious supply chain attack.  Kaseya VSA was a particularly attractive target as this is software used by Managed Service Providers (MSPs), a type of online contracting business.  Details of the story are still breaking and things may look different (and probably significantly worse) by the time people come into work on Tuesday or Wednesday.

So, what actually appears to be happening?