passwd- file

To: For users of Fedora Core releases <fedora-list@xxxxxxxxxx>
Subject: passwd- file
From: Nina Pham <nina@xxxxxxxxxxx>
Date: Tue, 20 Jul 2004 13:33:54 -0700
Reply-to: nina@xxxxxxxxxxx, For users of Fedora Core releases <fedora-list@xxxxxxxxxx>
User-agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.1) Gecko/20040707

I thought there's only passwd file, but today when I checked the system, I also saw passwd- file. What is that file?