A family of Microsoft word processing software products for creating web, email, and print documents.
Normal.dotm is what you would be looking for, and by default it will be in C:\Users<ProfileName>\AppData\Roaming\Microsoft\Templates. Note that it will not exist until you have made changes that need to be saved to it.
If you're looking to open it to edit it directly, here's a way to do it that doesn't require actually locating it:
In Word, press Alt+F11; the Visual Basic Editor opens. Press Ctrl+G to activate the Immediate window. In that window, type
NormalTemplate.OpenAsDocument
and press Enter. Normal opens, but note that it may not be on top of all windows. You can close the Visual Basic Editor. Make the desired changes to the Normal file, and then save and close it. If the changes you have made have not been recognized as changes (the Undo button is not activated), press Spacebar, then Backspace to "dirty" the file, thus ensuring that the template will actually be saved.