Is there any taxonomy/language to describe user interfaces?

Question

I'm currently researching the options which I have, to build an 'Online Help System'. This system should offer the user information about dialogs - which consists mainly out of forms.

So my main question is: Is there any taxonomy/language to describe user interfaces? I have searched a lot on this subject matter, but end up with things like XUL. Which is not what I'm looking for. I'm looking for a structure / language / taxonomy to describe the user interface (of the dialogs). However, I'm not sure of its existence. Since I'm not, I would also be pleased with any (search) terms which are worth investigating in this case.

Besides this, I'm also looking for languages / taxonomies which are used for regular Online Help systems. The things I came across were: DocBook, DITA and MAML. Are there any other big players in this 'market'?