Mark Krynsky at the Lifestream blog gives us a pretty good working definition:
Lifelogging is the process of tracking personal data generated by our own behavioral activities. While Lifestreaming primarily tracks the activity of content we create and discover, Lifelogging tracks personal activity data like exercising, sleeping, and eating. This may sound a bit confusing but hopefully the distinction between the two makes sense. The Quantified Self movement takes the aspect of simply tracking the raw data to try and draw correlations and ways to improve our lives from it.That page also lists over a hundred web services, apps, gadgets and sensors and more that can keep devotees on the tracking track. The sheer number of companies who think there's money to be made from this suggests that life-logging has become more than a fad and now has a growing customer base. I won't be surprised to learn that in a year from now that list has considerably dwindled.