GPX (or the GPX Exchange Format) is an XML format for storing GPS tracklogs. It is used by most consumer GPS devices including handheld hiking, cycling and car-mount navigation systems, and is supported by apps for most smartphones.
The GPX file contains "track points" which are individual readings of location combined with optional elevation data and a timestamp (in ISO 8601 format). Put together, these can be used to produce a visualisable trace which can be rendered on top of a map.
Apple's Aperture software allows you to add geographical metadata to images using a GPX trace by matching timestamps between the camera and the location in the tracklog stored in the GPX file.