Being able to read EXIF data is essential for every photographer.
EXIF (Exchangeable Image File Format) are tags embedded in photographs taken by digital cameras. These tags include information such as camera model and camera make as well as information about the settings used for each photograph. Things like shuttter speed, focal length, ISO speed and other relevent information is embedded in the image.
As a photographer it's a good idea to be able to access this information. Many image manipulation programs (for example, Photoshop & Lightroom) have utilities built in which allow access to EXIF data.
If you don't have Photoshop or Lightroom don't despair. There is a very small (but very effective plugin) for Firefox called FxIF which allows seeing EXIF data. It's very easy to install and works really well.
The screencast below walks you through some of the details behind EXIF as well as how to install FxIF to Firefox.