It depends on what you're trying to do. If you're trying to copy text and
formatting from one document to another, just use the clipboard, and Word
should do that by default.
If document A contains a set of styles that you want to use in creating a
new document B, you can create a new document based on document A, even if
it's not a template. Simply delete the unwanted/irrelevant text.
If document A and document B already exist, you have several choices. You
can save document A as a template, then attach that template to document B,
telling it to Automatically update document styles (in Word 2003, Tools -
Templates and Add-ins - Attach).
Alternatively, you can copy styles from document A to document B using the
Organizer. In Word 2003, Tools - Templates and Add-ins - Organizer - Styles
tab.