QoS (Quality of Service) is a complicated work in progress designed to increase the ability of real-time applications on the Internet. The Internet was designed as a "best-effort" network that does not differentiate its service response between different traffic streams. The idea behind QoS is to allow for different levels of bandwidth, delay variance (jitter), etc. These service levels will in effect augment the Internet to allow superior service levels, or predictable service responses independent or network conditions.
A widespread and end-to-end QoS deployment would allow for applications to provide services such as video conferencing, internet phones, streaming media, multicasting, conferencing, and much more.
The most common QoS protocols are ATM, DiffServ, IntServ/RSVP, and MPLS.