Remove Personal Information. This feature was new in Word 2007, and it strips out tracked changes and other metadata which you don't want customers, the media, or other people to see. Use this when you're sharing a final document with people outside your company. Click File, Info, Permissions, and then Inspect Document.
Prevent Editing. Do you want to solicit feedback on only certain parts of a document, and don't want people mucking around with your prose willy-nilly? You can easily restrict editing to just sections of doc, limit editing to certain document elements, or even only allow specific people to make changes. Click File, Info, Permissions, and then Restrict Editing.
Password Protect It. Do you have a sensitive document that you need to share, but it's critical that only authorized people see its contents? Not a problem. Click File, Info, Permissions, and then Restrict with a Password.
Share it as a PDF. Finally, if you have a "final" document that you want to share but don't want anyone to modify, save it as a PDF. Yes, PDF can be changed, but you need to really go out of your way to edit a PDF document, whereas it's possible to edit a Word document with a forceful sneeze. In Word 2010, you can just choose File, Save As and choose PDF from the list of file types.