A description or measurement of the overall performance of a network in controlling packet loss, bit rate, throughput, transmission delay, availability, jitter, etc., and managing network resources by setting priorities for specific types of data (video, audio, files) on the network. 

Quality of service is the ability to provide different priorities to different applications such as video on demand (VoD), Internet Protocol Television (IPTV), Voice over Internet Protocol (VoIP), streaming media, videoconferencing and online gaming.