Computer World explains how -
..the Exchangeable Image File format (EXIF) specification adds metadata to common JPG and TIFF image files. Along with a thumbnail image of the photo, EXIF data stores details about aperture, shutter speed, focal length, metering mode, and ISO settings....There's also room for other information, such as the camera's make, model, and registration number, and in some cases, location data.
...many newer digital cameras and mobile phone cameras have built-in GPS receivers. The geotagging features in these newer devices are integrated and seamless, and your EXIF files may store latitude, longitude, time (in the form of Coordinated Universal Time or UTC readings), and even altitude data (which can be helpful for reconstructing a family vacation on a map).
..once you start pulling on the thread of information contained in a geotagged image, a single photo can reveal a whole trove of personal data--far more than you might think.
Fortunately, smartphones and cameras let you turn off the photo geotagging feature.
Also see:
Face Detection & Tagging
Digital Photography Tips & Tricks