Since I'm getting more interested in photography and understanding cameras and techniques, I find myself wanting to know the details under which a photo was taken. Modern digital cameras encode a lot of such data — shutter speed, lens focal length, etc. — into the image file, generally called “Exif Data” (“Exif” stands for “Exchangeable Image File Format” and as an acronym would normally be written as “EXIF”, but the standard creators explicitly say that it should be written “Exif”, which is nice.).
That's just the summary — you can see the full data using the tool itself.
The amount of data encoded in the image is quite variable. Many times there's just about nothing, as the data is stripped somewhere along the way. Here's a version of the previous picture with most data missing. It's missing because it's a smaller version that's meant for web display, and for such use the data just makes the file bigger and slower to download.
Geoencoded photos get links in the summary area to Google Maps and the like, and below that is an embedded Google Maps pane. With either, you can switch between Satellite and Map, and zoom, etc..
You can also check images on your local hard drive — images directly from a camera generally have the most information. Give it a try!
If you're using Firefox or Safari, you have the added benefit that you can install an Exif-viewer button on your button-bar toolbar. Once you've done that, later, when you're viewing a page with an image you want to check out, just click the button and you'll be whisked to a new tab showing the image's data. I find this really useful. It doesn't work in IE, though, sorry.
I use the viewer a lot on images I see in the Digital Photography Review Samples and Galleries forum. Lots of nice pictures in there. Many have their Exif data stripped, but many do not.
Finally, I should note that my viewer makes use of Phil Harvey's most excellent Image::ExifTool library. Thanks Phil!