Quality of Service (QoS) is the ability to prioritise between different types of data stream on a network to guarantee a constant data flow.
For example, when making a VoIP call, voice packets need to take priority over data packets when travelling across the network. This is because voice is real-time-data and has to be played back at a constant rate, with no interruptions, to avoid poor quality or even unintelligibility.
It is, therefore, vital to maintain the constant flow of the voice packets so the sound quality of a call is not affected, to achieve this, the network must use QoS.
QoS distinguishes between voice and data, and gives voice priority in order to ensure a steady stream.