KML, which was developed for Google Earth, is a file that includes data associated with your location that can then be used when creating more interactive maps or trip logs. The feature is especially useful if you have background location, a new feature of Loopt 3.0 for iOS 4 [iTunes link].
Background location sharing, which is enabled via the new multitasking features in iOS 4, allows users to automatically keep track of their location changes so that they do not have to constantly check in or update their status messages.
You can customize who you want to share your location with (if you want to share it at all) and how often your location is updated and monitored. The really cool part of background location is that you can also get alerts if people you know (and who are sharing their info with you) are nearby.
By having the option to save your roaming history, in addition to what checkins you log, you have the potential to do some pretty cool things. For instance, you can create a map that pinpoints every place you visited while on vacation. Or, if you're a photographer and you want to geotag your photos, you now match the time stamps with specific location information.
Future versions will even let you attach other info or tidbits to the KML file, so if you attach a message or recommendation to a checkin, you can have that info integrated into the map file.