What exactly is Quality of Service? In general, on a network, QoS generally refers to measurables like latency and throughput, things that directly affect the user experience. By default network traffic, packets, are generally handled in a best effort fashion. However, if you have ever whitnessed your interactive Internet applications experiencing network delays, it becomes clear that best effort is often not good enough. Some flows need preferential treatment. Fortunately, the possibility exists to handle different flows of packets differently; to recognize that some traffic requires low latency or a rate guarantee for the best user experience. QoS is sometimes referred to as traffic control, or traffic shaping.
Traffic classification and shaping take place at the egress of a gateway router. To understand the benefits of traffic classification and preferential treatment, let's take an example. Let us a assume a basic router is setup with the default FIFO scheduler for best effort handling of flows. The pipe is at or near capacity and a queue forms. You can think of the queue as a line at the entrance to a toll booth on the highway. The toll plaza can only accept a limited number of cars simultaneously. Any additional travelers queue up in a lengthy line. Now, imagine an ambulance with a critical patient is enqueued at the back of the line. A packet scheduler can make the determination, based upon some given criteria, that this particular flow, the ambulance, is important enough to warp to the front of the line, allowing it quick passage. (Now, if only such a device could warp my vehicle to the beginning of the line at such toll plazas.) The above scenario is beneficial when an artificial queue is employed at the local egress point you have control over to thrawt an unfriendly upstream queue, such as at a broadband Internet service provider.
Ingress traffic, flows that are already upon you, are a different story. It is generally considered difficult to shape ingress traffic, since you have no control over QoS policy decisions outside your network infrastructure.