What is the Google "MayDay" update?

The label the SEO community have given some recent Google long-tail search algorithm changes. "MayDay" because it was spotted around the 1st May 2010. Below are some attributes of this change.

• Affects long-tail searches.
• An algorithmic change rather than a crawler or indexing.
• It's about how Google assess the best quality matches.
• Great content and authority are still valid.
• Nothing directly to do with the Caffeine update, side by side.
• Not a manual process, no humans involved, fully automated.
• Intended to be a permanent change.

Matt Cutts on the topic