Hello Xavdla,
webhooks will be triggered when certain events occur, see Webhook events for a list of possible events. Because multiple actions can trigger the same event it is difficult to say which action triggers which event. For example a product can be created through the Shopify admin web interface, but an installed App can also create products using the API.
Maybe you should see webhooks as a way to register your interest in changes to a particular object. For example if the product changes, you want to be notified.
When the event is triggered it will submit a POST request to the specified URL, the body of this POST request contains the 'XML/JSON data relevant to the event that triggered the request'. The exact XML/JSON data is also specified in the API docs, for example the Product Object contains the "Product Properties" section with a list of included variables.
Also note that there will be a time delay between the event and the actual POST request, it is not abnormal when this delay is 60 seconds.
HJ