IEEE 802.1p

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IEEE 802.1p, also known as Class of Service (CoS), is a 3 bit field within an Ethernet frame header when using tagged frames on an 802.1 network. It specifies a priority value of between 0 and 7 inclusive that can be used by Quality of Service (QoS) disciplines to differentiate traffic.
IEEE P802.1p is also the name of a task group active during 1995–98 responsible for adding traffic class expediting and dynamic multicast filtering to the IEEE 802.1D standard. Essentially, they provided a mechanism for implementing QoS at the Media Access Control (MAC) level. The group's work with the new priority classes and Generic Attribute Registration Protocol (GARP) was not published separately but was incorporated into a major revision of the standard, IEEE 802.1D-1998. It also required a short amendment extending the frame size of the ethernet standard by four bytes which was published as IEEE 802.3ac-1998.

Short Summary

802.1p defines eight different classes of service which are available, usually expressed through the 3-bit user_priority field in an IEEE 802.1Q header added to the frame. The way traffic is treated when assigned to any particular class is undefined and left to the implementation.
The IEEE however has made some broad recommendations:

User priority	Traffic Type
0	Background
1	Best Effort
2	Excellent Effort
3	Critical Applications
4	Video, < 100 ms latency
5	Voice, < 10 ms latency
6	Internetwork Control
7	Network Control

Note that the above recommendations were revised in IEEE 802.1Q-2005 and differ from the original recommendations found in IEEE 802.1D-2004.