Commonly known just as UHS, it is basically a file format (plus associated reader software) for providing walkthroughs for video games in a more useful format.
It is based around the concept of having sets of questions (usually organised into categories, usually one category for each part of the game) and having several sets of answers to each question, one providing more information than the one before. The idea is that, if you're stuck, you can just obtain the level of information you require, which avoids the hints from spoiling more of the game than is needed.
The system is closed, however; authored hint files must be compiled before they can be used in the viewer, and this requires sending them to the author of UHS for review. I quote from their current website, "Please remember that the UHS is not just a generic file format. We are essentially an on-line publisher that happens to use a common file format to distribute its content". They do tend to pay authors for the files they create.
Newer version of the software have added support for embedding images and audio inside the hint data files. You can find the software and data files hiding at www.uhs-hints.com, which also provides a means of reading most of the data files online, with no need for downloading the hint files or installing a reader.