QoS addresses quality metrics. Each affects applications differently.
Latency is end-to-end packet delay. Components include serialization, propagation, and queuing delay. Voice requires under ms one-way.
Jitter is variation in latency. If packets take ms then ms, jitter is ms. Applications buffer to smooth jitter, but buffers add latency.
Packet loss occurs when queues overflow. TCP retransmits lost packets. UDP loses the data. Voice tolerates % loss. Video degrades past %.
Your QoS strategy must address all metrics.