Check out this article for a detailed discussion.
http://www.fontstuff.com/access/acctut08.htm
A brief quote from an John Vinson (Access MVP) posting.
You'll need to create a small unbound Form (let's call it frmCriteria) with
a Combo Box control (cboCrit) on it. Use the combo box wizard to select the
table for the selections, and be
sure that the bound field of the combo is the value you want to use as a
criterion. Save this form.
Now use
=[Forms]![frmCriteria]![cboCrit]
as the criterion in your Query.
It's convenient to base a second Form or Report on the resulting query to
display the results; if you put a button on frmCriteria to launch that form
or report, the user can enter the criterion and view the results in one
simple operation!
Quoting John Vinson
Keywords: Parameter Queries