Submit button on the inside of the field - Good or Bad?

1. Does the submit button need to be inside the form?
2. What is difference between button and submit?
3. Which button must be included in every form?

Does the submit button need to be inside the form?

You can tie a submit button to a form that the button doesn't live inside of. The trick is to give the form an id and then reference that id with the button's form property.

What is difference between button and submit?

A 'button' is just that, a button, to which you can add additional functionality using Javascript. A 'submit' input type has the default functionality of submitting the form it's placed in (though, of course, you can still add additional functionality using Javascript).

Which button must be included in every form?

The submit button is defined by the input type="submit">, which submits all form values to a form-handler. In most cases, the form-handler is a server page with a script for processing the submitted data. The form's action element specifies the form's handler.