We would like to create a Help for a Touch Screen application.
How different is this to creating a regular HTML or WebHelp?
If there are differences, please tell me what I need to do?
Quite apart from the terminology differences (e.g. click versus tap) the differences largely depend on the devices on which the application will run. Each have their own characteristics (e.g. screen size, resolution, etc) which need to be taken into account. The recently released RoboHelp 10 allows you to output for mobile devices and you'd be best to look at the collateral released around this. Check out these links:
The applications for which we are looking to write the Help is a touch screen application running on a 23 inch HP monitor. Do you know if the output for mobile devices available in RoboHelp 10 will cater for this?
RoboHelp 10 HTML5 output can indeed help you. Since your application runs on a 23" monitor, you may need to test whether the supplied default layouts suit your needs. Perhaps you can simply use a default layout but it is always possible to design and build a layout specifically catered to this desktop.