ExifTool GUI, as it name implies, is a (free) Graphical User Interface for an already existing EXIF editor called ExifTool, developed by Phil Harvey, a full-featured - command-line - utility. ExifTool GUI was devised to help you make the most of that tool without the hassle of having to type instructions and their corresponding parameters.
Both tools complement each other perfectly. This GUI is just a different way of using most of the features already implemented in ExifTool. In fact, it adds no new functionality to the viewing and editing facilities already available in Mr. Harvey's program - it just makes it easier for those not too familiar with the DOS environment. And the results are stunning.
The interface shows you not only the longest list of EXIF tags I have come across so far, but also those IPTC (copyright-related information) and XMP tags (adjustments made with your camera to your RAW photographs) that are most commonly used. To make things even easier, your photographs are shown as thumbnails, helping you to easily match each set of tags with the image they belong. All editable tags will appear in blue in the tags panel, allowing you to edit them individually or to apply specific changes to a batch of selected images. Different types of tags will open different editing windows, though you will be allowed to transfer some XMP metadata to the IPTC tag set. Once edited and corrected, all metadata can be sent to an HTML file.
Finally, you are also provided with a useful "Remove Metadata" utility, which you are advised to use carefully. Here you can delete ALL metadata or groups of them belonging to each category (EXIF, IPTC, and XMP). Thankfully, this powerful feature comes with a "back up" box that, when checked, will let you undo any accidental deletion.