I don't find those descriptions from within workflow to be particularly helpful. The real answer is that they are both similar, and that the Web Request is the more general option of the two. By default, the Web Request activity is set up to be a HTTP GET request, but it is capable of making POST, PUT, or DELETE requests as well. The Form Post activity is set up by default to be a POST request, with the option of changing it to a PUT request. I'll leave it up to you to research the differences between the various HTTP request types.

The Web Request activity is more flexible and customizable. If you wanted, you could set up a web request to POST in the same way as the Form Post. However, you are also free to structure your POST or PUT requests in any way you like, as it provides an editor and a blank textbox. The Form activity is set up to be easily configurable to insert a series of (name, value) pairs as a form submission only.

Bottom Line: It's probably more convenient to use the Form Post tool if you're just making a simple form submission, rather than writing your own POST request in a Web Request activity. If you are only trying to GET information from a resource, then the web request is your only option.