Quality of service (QoS) is the overall performance of a telephony or computer network, particularly the performance seen by the users of the network.
Above is the Quality of Service definition from the Wikipedia. Performance metrics can be bandwidth, delay, jitter, pocket loss and so on.
In this post I will not explain each method or special tools used in each method. Instead, which method makes sense in particular design and which Quality of Service Tools can solve user needs without compromising the network design goals.