EXIF is the EXchangeable Image File format and is used by digital cameras to store information about the photos you take. Everything and the kitchen sink (figuratively speaking of course) is stored every time you take a photo.
Here are two examples of EXIF data using the Flickr service:
You’ll notice that the two photos differ significantly as far as the EXIF data is concerned. In addition to the time and date, the make/model of digital camera is given along with all available photo information as well. In 2005 (the first photo), I had an Olympus D-535 camera. The second was taken recently with my Fujifilm Finepix A820. Two different cameras, two totally different sets of settings based on environment and what my point-and-shoot digital cam "thought" was best for each shot.
Both photos are also geotagged, meaning they both contain GPS coordinates stating exactly (more or less) where each photo was taken.
To note: No, I don’t have one of those super-duper-pooper expensive digital cameras that records GPS positional information. Rather I manually enter in positional information using Flickr’s map function using point-and-click or entering manual coordinates.
Concerning your privacy
With online photo services like Flickr and Picasa, you can opt whether or not to have this information shown.
Maybe you have a super-secret way of taking photos and don’t want people to know how you took specific shots. Maybe you simply don’t want people to know when you took specific photos.
Hiding the EXIF data is the way to do that.
Concerning the geotagged locations, the vast majority of digital cams are not GPS-enabled (and if yours is, trust me, you would know).
Flickr does have privacy options to only let specific people (i.e. your "friends" list) know where shots were taken if you geotag. Or you can simply not geotag photos at all.
I’m not familiar with Picasa’s way of geotagging privacy but if anyone has info on that, feel free to chime in with a comment on their options.
Also remember: If you e-mail a photo to someone direct from your digital cam, all the EXIF data is present within the file(s). This information can be seen even in the standard Windows Explorer as a column. If you don’t want this information shown for whatever reason, you need to bring the file into an image editor and re-save it to remove the EXIF data.
