HTML forms are a fundamental part of building interactive websites, enabling the collection and submission of user data to a server for processing. An HTML form is created using the <form> tag. The action attribute of the <form> tag is a URI that points to the webpage that will process the submitted form data. The method attribute specifies the HTTP method employed to send the form data. The two most common methods are GET and POST.

With the GET method, form data is appended to the URL in name/value pairs. This method is typically used for fetching documents or data. However, it's not suitable for sensitive data, as this data will be visible in the URL. Conversely, the POST method packages the form data in the body of the HTTP request. It is commonly used for updating data and is more secure, as the data isn't exposed in the URL.

Forms can include a variety of input elements, defined using the <input> tag. The type attribute of the <input> tag determines the kind of input field to display. Common types include 'text', 'checkbox', 'radio', and 'submit'. Furthermore, the 'required' attribute can be added to an input field to make it mandatory for the user to fill out before submitting the form.

Here's a more advanced example of an HTML form that includes different types of inputs, along with the 'required' attribute:

<form action="/submit_form.php" method="post">
  Name:<br />
  <input type="text" name="name" required /><br />
  Email:<br />
  <input type="email" name="email" required /><br />
  Gender:<br />
  <input type="radio" name="gender" value="male" /> Male<br />
  <input type="radio" name="gender" value="female" /> Female<br />
  <input type="submit" value="Submit" />
</form>

In this example, the form includes text, email, and radio inputs. The 'required' attribute is used, which means the user must fill out these fields before submitting the form.