In the VoIP / telecommunications field, the term QoS (Quality of Service) refers to resource reservation control mechanisms. Quality of service is the ability to provide different priority to different applications, users, or data flows, or to guarantee a certain level of performance to a data flow. Quality of service guarantees are important for VoIP since VoIP is real time and requires fixed bit rate. QoS mechanisms are not required if there is no network congestion.
For more information on QOS, refer to the Cisco Internetworking Technology Handbook chapter called Quality Of Service (QoS).