Bare with me if this question is plain stupid...
When I talk to an developer from the Microsoft ASP.NET world and he uses the word "Webservice", does that word in every case imply a specific data format (XML? SOAP?)?
Or is it just anything you can call via http(s)?
The term Webservice itself is language-agnostic.
This is a decent overview.
In my view, it can be anything that's over http/https, and intended for calling by an application rather than a user's browser.
In particular, REST and SOAP are quite different about how they pass arguments in and get results back
If an Asp.Net developer says WebService, you can pretty much bet that they are talking about XML/SOAP.
However this is not universally true. I think it's just fine to call anything a WebService if 1) the data source is available via the web or 2) it is a web address that can provide back information given a set of inputs.
For example, StackOverflow.com allows for screen scraping of the User pages in order for 3rd party applications to be built. It's not specifically XML/SOAP but I would consider it a Web Service (format #1)
In my experience this completly depends upon who you are talking to. For some ASP.Net developers this is only SOAP for others it includes other things like REST. If you are planning on using the term in a specification it would be a good idea to be a bit more specific.
I can only agree with Paul, anything queried over the web, using the http(s) protocol and not browser oriented. But any web service should also have the functionality of being discovered (WDSL and so on).