In my app I get the form HTML of a website. That form displays customer information. Now I can parse the form and get customer information. Or I can use a web service (SOAP or whatever) and get the same information? What is the difference?
On a general level, a website is displayed to a user in a browser making it consist of GET request through HTTP/HTTPS. A webservice is one that responds to more than just GET requests (eg. POST, PUT, DELETE). This means that a user can interact with data remotely. Like adding your post to stackoverflow.com, that meant you sent a POST request where stackoverflow can store the data contained in this post in some remote location, like a database.