QoS stands for quality of service, and is related to transport of traffic with special requirements, namely those involved with telephony and computer networks. The term was defined by ITU in 1994, focusing on all aspects of a connection, such as service response time, loss, signal-to-noise ratio, cross-talk, echo, and other key facets related to the overall quality of a call. The end goal, per se of QoS, is to achieve and maintain a level of performance, despite negative factors that would influence a network otherwise.

In the realm of VoIP, QoS falls under computer networking, as it relies on packet-switched telecommunication. Quality of service, more or less, is the ability to provide priority to certain applications in an effort to guarantee a certain level of performance or data flow for a user.  Prioritization of network traffic is implemented to insure that networks are performing at a desired level. Getting into specifics, a required bit rate, delay, jitter, and/or packet dropping probability may be guaranteed. Quality of service guarantees are especially important if network capacity is insufficient, namely in the case of real-time streaming multimedia such as voice over Internet protocol.

QoS is essentially damage control, and a means to monitor achieved level of performance, proceeding to dynamically control priorities within a network. Quality of service is used as a preemptive measure, and it's name alone is enough to spur on confusion. High QoS is often mistaken as meaning a high level of performance or achieved service quality, such as high bit rate, low latency and low bit error probability. The is untrue, as Quality of service is a mechanism – a set of techniques to manage network resources. If a network is incapable of handling a certain level of data, then QoS is rendered moot and ineffective. 

VoIP relies heavily on QoS, which focuses on uptime, bandwidth, latency, and error rate to deliver communications free of jitter, echo, and other undesirable interference. Quality of Service is in demand across more corporate environments and private networks, which rely on high levels of performance with minimal, if any error.

Once again, QoS is a means of prioritization, so if your ISP provides QoS favoring voice, your voice reception would be optimal while other media types could suffer. Qos is an essential tool for VoIP success, and as more advanced versions of this technology become available to end users, voice over IP communications are nearing perfection. Of course, the quality of the actual Internet speed is a prerequisite to making QoS work. Using a bandwidth calculator prior to committing to VoIP is a smart practice, as it will reveal the demand of your network or if implementation of VoIP is possible at all.

See Also:
Fault Tolerant Design
What Kind of Quality Can I Expect Using a Business VoIP Phone System?
How Packet Switching Works