What tool/service would you recommend to describe the functionality and conceptual model for a web service?

Question

Project management

The bottom line is this: it is planned to develop a web service and first describe its logic, functionality and provide some final concept.
You can, of course, do everything in Word, but there are probably more interesting tools.
Ps: it will still be necessary to write the TK, but this is probably exactly done in Word. Yes? :)
UPD: highly desirable for free and under windows

Answer 1

https://www.mindmeister.com

Answer 2

I use Omnigraffle - it's very versatile and allows you to quickly create charts, mind maps. For simple projects I use mindnode .
In small projects, with the correct construction of the concept map, often (not always) a separate description of each module (page) is not required. In the TOR, it will be enough to indicate the name of each module (page) according to the concept.
In "larger" projects, to describe the functionality and logic of each module, you can use the same programs for building concept maps or powerpoint.
Word has not been canceled either, but in practice, the more visually everything looks on paper, the fewer questions later.)