A textbox is a distinct object within the document object model; a frame is
a positioning and formatting instruction applied to some part of the body of
the document. Textboxes are a later development in the history of Word. For
most purposes they are easier to use and have more formatting options. If
you simply want to position some text somewhere on the page, you can use
either.

A textbox always contains some text -- if only a single empty paragraph to
which a graphic is anchored.

Textboxes can be combined with other textboxes and drawing shapes to create
composite graphics.

The content of a textbox is not part of the MainStory range of the document,
so it's not included in the TOC. Use frames if you want to position your TOC
headings in unusual ways.

Thorsten,

Further reading from Microsoft that you may find helpful:
Re Word 2003
http://office.microsoft.com/assistance/preview.aspx?AssetID=HP051893331033&CTT=1
&Origin=EC790000701033&QueryID=LAo4-XfGK&Query=frame&Scope=HP%2cHA%2cRC%2cFX%2cX
T

Re Word 2000
http://support.microsoft.com/default.aspx?scid=kb;en-us;211954&Product=wrd20

Jon

A text box is in the drawing layer. Word cannot "see" text in a text box for
purposes of TOCs, TOFs, cross-references, etc. Nor can you see a text box in
Normal view. A frame is in the text layer and can be viewed in Normal view
(though not in position). It can also be part of a paragraph style (which a
text box cannot). For more, see “WD2000: General Information About Floating
Objects” [KB 268713]” at http://support.microsoft.com/?kbid=268713

--
Suzanne S. Barnhill
Microsoft MVP (Word)