A common element present in web pages is forms. Forms are a way of sending values into a web page, for example, to create a new comment on a blog post, or to submit a purchase.
Browsers present the forms so you can input values and send them in a single action after pressing the submit or equivalent button. We'll see how to create this action programatically in this recipe.
Be aware that sending data to a site is normally more sensible than receiving data from it. For example, sending automatic comments to a website is very much the definition of spam. This means that it can be more difficult to automate and include security measures. Double-check that what you're trying to achieve is a valid, ethical use case.