There's no particular software that I have in mind to try and implement this but if it's possible in MS Office or CS6 (or some freeware) I have those available.
Basically, I want to make it so that within a document I can add a hyperlink to a word or phrase the, when you click on that hyperlink a text box pops up (preferably to the side or over the top of the text, rather than in-line) with details.
This is going to be used for notes for an RPG, where I can click on a skill or action's name and up pops a box with its rules. So, best-case would be to call the same text box in multiple locations but copy & pasting to multiple boxes isn't too bad, as long as I don't have to add to pre-existing definitions.
I had considered doing this in a web page but I'd like to aim to do this in a document if possible in case I want to do it with other things too.