Garmin Foretrex is a small GPS designed to be worn on the wrist, similar to Forerunner. Although they look nearly identical to Forerunner 101 / 201, the firmware built into Foretrex is very different. Foretrex is a general purpose GPS and doesn't have any specialized features for running or biking (e.g., run history, lap / pace data, etc.).
If running or biking will be your primary use for GPS, you will probably want one of the specialized GPS models like Garmin's Forerunner or Edge.