Exif data is a specification in the photo used by Digital camera. Every information like Camera model, Manufacturer, Aperture, Shutter speed, ISO, etc. are stored in Exif in the photo. You can check basic information of Exif data in your photos in Windows XP and later version of Windows, of course Mac supports as well.
I wrote about how to check shutter count in your camera with Opanda before and it works great because it shows every information from Exif, but the problem is that it works on Windows only. But if you’re Mac user, you can try Simple EXIF Viewer. I stumbled upon a web site, Jeffrey’s Exif viewer that can easily check tons of Exif information in photo on the fly without installing any of those software. You just need to give the photo url from web or upload the photo.