More than I want

Try to place an AutoShape on a page and the Drawing Canvas pops up by default.

To turn off this feature, go to:
Tools>Options.
On the General tab, remove the check mark from
"Automatically create drawing canvas when inserting AutoShapes"

To just dismiss it each time, choose your AutoShape and then touch the Delete or Esc key before drawing the object.

Here's some more information.

Wordsite.com:
The draw layer: a metaphysical space

Knowledge Base
General Information About Floating Objects
(a discussion of Word's floating objects and layers)

As I understand it, the Drawing canvas is not really a new layer. The following illustration shows the classic layers. It is from the Knowledge base article:

How to Place Text over a Graphic

                                           ___________________
                                          /                   /
                                         /   <SURFACE OF     /
                                        /       PAPER>      /
                                       /                   /  /
                 Front drawing layer  -------------------  /
                     MAIN TEXT LAYER  =================== / /
                  Back drawing layer  -------------------/ / /
                                                          / /
                 Front drawing layer  -------------------/ /
   (Header/footer) BOTTOM TEXT LAYER  =================== /
                  Back drawing layer  -------------------/

Labels: Graphics, Tips