On an unbound form, put a textbox for the ID entry.
Add another textbox for the description.
In the afterupdate event of the first textbox, put this code:
Private Sub Text0_AfterUpdate()
If IsNumeric(Me.Text0) Then
Me.your2ndtextbox = DLookup("yourdescriptionfield","yourtable",
"yourtableID = " & [yourfirsttextbox])
End If
End Sub
I am assuming the ID field is a number, by the way....