Metadata
Metadata is "data about data". This means every time you take a picture with your digital camera, you are recording a lot more than the image. In fact, as digital cameras (or imaging devices in general) progress, the range and diversity of types of data grows as well.

Here are some of the different kinds of data you can record within an image's metadata. These are broken down how Adobe Photoshop breaks them up.

File Properties
File Name
Document Type (jpeg, tiff, psd)
Date Created
Date Modified
File Size (megabytes)
Dimensions (width vs. height)
Dimensions in Inches
Resolution (pixels per linear inch)
Bit Depth
Color Mode (rgb, cmyk, grayscale)
Color Profile (sRGB, Adobe RGB)

EXIF (Exchangeable Image File Format)=Camera Data
Exposure Mode (manual, aperture priority, auto, etc.)
Focal Length (28mm, 105mm, etc... how zoomed in or out you are)
Lens (range of zoom available, ie. 28-135mm)
Aperture (f2.8, etc... what it was set to when you shot)
Shutter Speed (1/2000 of a sec., what it was set to when you shot)
Max Aperture (biggest available aperture on the lens you used)
Flash (fired, didn't fire, auto, ttl, etc.)
Metering Mode (spot, partial, center-weight, evaluative)
White Balance (auto, snow, tungsten, 6500 Kelvin)
Camera (Canon Eos 40D)
Serial Number

IPTC (International Press Telecommunications Council)=Added Stuff
This can be almost anything these days...
Author
Copyright
Country
Geo-Tag (longitude and latitude)


Viewing
To view metadata, you need a program that can make sense of the data such as adobe photoshop's "bridge" program. If you don't have access to the Bridge, then look at the software that came with your camera. It should have the ability to view at least some of the metadata, if only the settings you shot the image on.

Some programs, such as Google's Picasa, make you look at the image's properties to see the metadata. There tends to be very little information, but at least it displays basic file and camera settings.

Devices
As metadata becomes more available and customizable, the creation of devices to utilize that data becomes more available as well. One of the more recent device trends is in what is known as geo-tagging. This is a useful device, camera, or card feature for landscape photographers, hikers, wilderness athletes, or foreign correspondents who want to tag an image with the coordinates of where it was shot in longitude and latitude. This allows them to revisit where they shot the image or use it scientifically, geologically, or as proof of a news story.