This appeared in Mac OS X 10.7 (Lion). It appears to be a tool which is invoked by Safari or System Preferences when Safari bookmarks changes are to be uploaded to iCloud. Presumably it also gets invoked from the cloud when necessary, for changes in the reverse direction. It is launched on demand, when there are changes, and terminates when its syncing work is done, typically in a few seconds when, say, a bookmark is added. However, if hundreds or thousands of bookmarks are changed, or for an initial sync of such bookmarks, it may run for a few minutes.