Where appropriate, back tabbing via Shift+Tab does the reverse of forward
tabbing - both in and out of Word. In Word, however, the action of the Tab
key is context sensitive - in some circumstances, user-configurable - and it
may do one of many things which may or may not have a logical opposite
action.
Regardless of settings, if the Tab key gives you a tab character, then
Shift+Tab in the same circumstances will also give you a tab character.
If you have the options Stefan mentions set and Tab gives you an auto
indent, then back tabbing will 'auto outdent' - that is, the reverse of
forward tabbing.
If you don't have formatting marks displayed you may not, however,
immediately notice the difference in the above two situations.
If you are in a table, on the other hand, the Tab key moves the selection
forward to the next cell and the back tab action moves back to the previous
cell, again the reverse of forward tabbing and, again, regardless of any
options.