QOS - Quality Of Service - is a networking feature that provides for protection of bandwidth, e.g. to ensure you have enough bandwidth to stream a video without dropout. Vista provides policies to enable you to prioritise network traffic by configuring Policy-based Quality of Service (QoS).
See http://www.microsoft.com/downloads/details.aspx?FamilyID=59030735-8fde-47c7-aa96-d4108f779f20&DisplayLang=en for more details on how to implement QOS with Vista (and Longhorn!).