The service is illustrated and described formally here:
To access this service (JSHTML v1.1), use the following serviceURL:
How to Create a Custom Search Page
If you are interested in placing a custom search environment into a web site, start by looking at the Template and Examples, which can be used to begin your own custom pages. You should also see the Documentation, which contains detailed information about how to use the service API including descriptions of each of the options that are available to control the features and behavior of your search page.