This is a possible update based on measurement tools that showed relatively sharp changes in Google's results pages. Webmasters on Webmasters World forums also reported changes, most of them for the worse. Some have even noted that they have seen more sponsored results in high places at the expense of organic results.
What to do if you were damaged by the update?
Since it is unclear exactly what the nature of the update was, there is not much you can do. However, since this is probably one update from a sequence of quality updates, pay attention to both the relevance and the quality of the text. In other words, the text you wrote may be excellent, but if it doesn't answer what users are looking for, Google may assume that the page is irrelevant.
And, of course, the opposite is also true, relevant text that is written in a bad way, may hurt you.