Regularly recurring events. Some events take place on a regularly recurring basis, such as annual conferences like [ICALP] or an event like the [presidential election]. Without specifying with your keywords, it’s implied that you expect to see the most recent event, and not one from 50 years ago. There are also things that recur more frequently, so now when you’re searching for the latest [NFL scores], [dancing with the stars] results or [exxon earnings], you’ll see the latest information.
Frequent updates. There are also searches for information that changes often, but isn’t really a hot topic or a recurring event. For example, if you’re researching the [best slr cameras], or you’re in the market for a new car and want [subaru impreza reviews], you probably want the most up to date information.
There are plenty of cases where results that are a few years old might still be useful for you. [fast tomato sauce recipe] certainly saved me after a call from my wife reminded me I had volunteered to make dinner! On the other hand, when I search for the [49ers score], a result that is a week old might be too old.
Different searches have different freshness needs. This algorithmic improvement is designed to better understand how to differentiate between these kinds of searches and the level of freshness you need, and make sure you get the most up to the minute answers.
Update 11/7/11: To clarify, when we say this algorithm impacted 35% of searches, we mean at least one result on the page was affected, as opposed to when we've said noticeably impacted in the past, which means changes that are significant enough that an average user would notice. Using that same scale, this change noticeably impacts 6 - 10% of searches, depending on the language and domain you're searching on.