It sounds like a combination of the glossary package (expanded to include URLs) plus some insertion into the posted content when content is first entered into the system, to wrap infobutton URLs around text that is recognized.
There are some JavaScript packages that do this as well, I think, but I doubt they pull much information from a database.
You would need to distinguish between unambiguous words/phrases and ones that could be ambiguous. For instance the word "psoriasis" only has one meaning and should probably be linked to the same set of info regardless of the page it is found on.
However, "twins" might refer to identical or fraternal twin children, or could be used as a figure of speech. You would want to have some method of determining when it would be appropriate to link to the "twins" URL.
