I would like to create a button that the user can press on the MS word toolbar, after this the document text is passed to a remote service, and some tabular data is passed back and displayed to the user.
Which would be the easiest an quickest for creating this? Can a macro do this? Or to display a button in the toolbar I must have an addin?
Sure a macro can do this. Adding a macro button to your toolbar:
http://web.ticino.com/multilingual/creating_a_macro_button.htm
Passing data to a remote service - well, you did not mention the kind of protocol you have in mind, here is an example of how to post a http request by using VBA:
How can I send an HTTP POST request to a server from Excel using VBA?