Yes, Inserts a 'comment' which will not display on the browser screen, but can be viewed in html file itself when you inspect its source using <!-- -->
No, Although they will works in all browsers, because the closing tag is considered optional. But never rely on this. It might produce unexpected results and/or errors if you forget the end tag.
The elements with no content are empty elements. They do not have any end tag, for example < br > element used for line break.
Html tags are keywords(tag name) surrounded by angle brackets.