Create the list  of items  to appear in the dropdown list in a range (could be on
another sheet.) Select all the cells  in the range, then assign the range a name
(Insert >> Name >> Define). Assume you named the range TheList. Select the first
cell which should display the list as a dropdown box. Select Data >> Validation.
On the Settings tab of the Data Validation dialog that appears, select List in
the Allows dropdown box. In the Source textbox, enter =TheList and click OK.
This creates the dropdown based on the named range. You can copy & paste this
cell; wherever you paste it will have the same dropdown list. This also lets you
maintain the list in one place: the named range (TheList).

Thanks for the clear explanation. As usual, I learned
something new from ExcelVBA today!

I believe you can also select a range of cells  (including an entire
column / row) THEN do the select Validation setup from the menu.

Selecting Validation from the menu should bring up a mini-wizard
where you can select the list-range, default cell as well as error
messages should the user violate your guidelines.