They are a set of building blocks for web applications that have been designed by user interface specialists, thoroughly thought through and usability tested. They can be used for developing full web applications, allowing designers and developers to focus on their application’s particular needs rather than the design of all the controls and elements inside.

The corresponding Woodstock toolkit for which they were written has been available under an open source (CDDL) licence for a while, but of course the guidelines themselves can be applied to any web app. (They do have a bit of a system administration app slant, though, for obvious reasons.)