Quality of Service, or QoS, is a set of networking techniques that control how traffic is treated so important applications receive predictable performance. It manages delay, jitter and packet loss by classifying flows, marking them with priorities, and then applying rules at each device that forwards the traffic. Typical uses include giving voice calls and live video a higher priority than large file transfers, so conversations stay clear even when links are busy. QoS does not create extra bandwidth; it decides how to share the bandwidth you already have when there is contention.
In practice, devices identify traffic by application, address or port, then mark packets, often with DSCP at the IP layer or 802.1p priority at the Ethernet layer. Routers and switches place packets into queues and schedule them with methods such as strict priority or weighted fair queuing, so time sensitive traffic leaves first while other classes share the remaining capacity. Shaping smooths bursts by holding packets briefly to match a chosen rate, while policing enforces limits by dropping or remarking excess packets. Congestion avoidance features like random early detection and explicit congestion notification help prevent queues from overflowing.