Exif data is the information your camera writes to your memory card for each image you take (metadata). The information becomes part of the image file. The most common Exif data categories are, camera model, ISO, shutter speed, aperture, focal length, day and time, and whether flash was used or not. There are many, many other Exif data categories such as which lens was used, distance from subject, copyright information, what software was used for post processing, and even various adjustment settings in that software, etc. Many cataloging software programs allow you to search through your images based on Exif data categories, so it can be of some use for finding images.
Personally, I’m a fan of Exif data and publishing it along with my photos posted to the Internet (if the site displays it). After I view a shot of someone else’s that I really admire, the next thing I want to know is what ISO, SS, aperture, and focal length it was taken at. To me, this is so helpful in understanding what the conditions must have been for the photographer. It’s another lesson in photography for me when I can tie this information to the image. I’m quite disappointed when good images don’t display their basic Exif data for the public to see.
I post basic Exif data for all of my gallery shots on my website, DennisDavenportPhotography.com.