This is a service for looking at what's changed in OSM where you're interested in. Rather than calculate the bounding box of all changes in a changeset, this service attaches changes to small lat/lon tiles, which are more fine-grained and more easily queryable (in theory, anyway...)
On the bottom left of the maps should be a control to allow you to select an RSS feed which will contain only those changes touching tiles actually in that box. So no more sorting through (big) changesets to find which actually modify features near you!
Click on one of the links above for a daily, weekly or forever view of the data. Initially, you may be too zoomed-out to see any data and there will be a warning in the lower left corner of the screen telling you to zoom in to see data. After zooming in you should see lots of orange boxes, each of which is clickable.
Clicking on one will take you to a list of changes which have occurred to elements within that box and there are links from there to view the element or changeset in OSM, or to view the changeset in OWL. The changeset view in OWL shows orange boxes covering the changes in that changeset.
Finally, any map view should have a link in the lower left which allows you to drag a bounding box over the map - this will make a link to an RSS feed appear in the lower left. The RSS feed contains information on the changesets which intersect the area you dragged, meaning you can easily keep up-to-date with the editing activity in areas you're interested in.
The graph below shows how up-to-date the data is, relative to the minutely diffs at planet.openstreetmap.org.