Here you will find documentation on the file formats used in Garmin devices as well as sites that integrate Garmin devices in data transfer, such as MotionBased.
General introductions, FAQs, and schema documentation are provided for the various formats.
Keep in mind that some of these formats are Garmin specific (i.e. AXM, TCX), while other formats were not created by Garmin yet can be used by Garmin devices (i.e. GPX, KML).
Activity XML (AXM) is a Garmin format for exchanging Activity Data (usually recorded from sensors) that provides the ability to transport just about any measurement that describes an Activity.
The Garmin Device Descriptor is a Garmin format provided by USB mass storage devices to describe the capabilities and details of extracting information from a Garmin device.
GPX (the GPS Exchange Format) is a light-weight XML data format for the interchange of GPS data (waypoints, routes, and tracks) between applications and Web services on the Internet.
GPXX (Garmin GPX Extensions Format) is an XML data format that extends GPS Exchange Format (GPX) using the extensions element. This format is Garmin-specific and provides more structured data for GPS data (waypoints, routes, and tracks).
KML is a file format used to display geographic data in an earth browser, such as Google Earth, Google Maps, and Google Maps for mobile.