This is all under the Review tab.

The space taken by the comment balloons squeezes the rest of your page into a smaller space. It doesn't hide anything in my experience. I personally stopped using comments when Word shifted to the balloon interface because it interfered with layout in print view.

If you right-click on a comment, you can delete it. You can also hide comments, but doing so doesn't hide them from others reading the document on their machines with their settings; it only hides them from you.

If Word is a part of your life, I would recommend sitting down with a book like the Microsoft Word 2007 Bible by Herb Tyson for a couple of hours. It will provide immediate answers to questions and make your experience more rewarding.