The Zhou dynasty 周 (11th cent.-221 BCE) was, in spite of its bad political performance from the 8th century BCE on, the longest-reigning dynasty of Chinese history and even worldwide. It was founded during the early 11th century BCE and found its end in the mid-3rd century BCE.
The kings of the Zhou bore the title of "Son of Heaven" (tianzi 天子), which was inherited by all Chinese dynasties. Although the term of the rulers of the Zhou, wang 王, is commonly translated into English as "king", some Western historians speak of the "emperors" of the Zhou, which is in fact not so strange, because in the late Zhou period, the regional rulers (zhuhou 諸侯) began adopting the title of wang, too. All the more, the rulers of the semi-"barbarian" states of Chu 楚, Wu 吳 and Yue 越 from the beginning called themselves wang. In order to discern between these "usurpatorious" wang and the proper "Son of Heaven", the term "emperor" might be quite useful. I will nevertheless use the traditional translation of "king", yet, in the one or other place use the term "empire" instead of "kingdom", when referring to the whole area the Sons of Heaven had an influence on, in other words, China of the 1st millenium BCE, or, as the Chinese termed it, "All under Heaven" (tianxia 天下).
The long rule of the Zhou dynasty is usually divided into two parts, the first covering the time when the kings of Zhou were the undisputed political leaders of the empire, and the second representing the time when the regional rulers, descendants of the former vassal states of the Zhou, became so powerful that the kings of Zhou, although acting formally as the highest arbiters, were politicall fully dependant of the activities of the larger regional states.
The first part of the Zhou period is called the "Western Zhou" (Xizhou 西周, 11th cent.-770 BCE) because the main royal seat was located in the west, near modern Xi'an 西安, Shaanxi), while it was shifted to the eastern capital near Luoyang 洛陽, Henan, during the second part of the Zhou period that is therefore also known as "Eastern Zhou" period (Dongzhou 東周, 770-221 BCE). The Eastern Zhou period itself is crudely divided into the so-called "Spring and Autumn period" (Chunqiu 春秋, 770-5th cent. BCE) and the period of the "Warring States" (Zhanguo 戰國, 5th cent.-211 BCE).
During the Spring and Autumn period some of regional rulers were able to become protectors of the house of Zhou against "barbarian" tribes from the north and of the traditional order. These powerful lords were called the "five hegemons" (wuba 五霸). The ancient order of the kingdom finally disintegrated in the 5th century, when some vassals of the regional rulers usurped power within the regional states. This was the family Tian 田 in Qi 齊, the Three Huan families (San Huan 三桓) in Lu 魯, and the viscounts of Zhao 趙, Wei 魏 and Han 韓 in Jin 晉.
While during the Spring and Autumn period the kings of Zhou were still respected as Sons of Heaven, they were nearly irrelevant during the Warring States period. The "Warring States" were powerful regional states that waged war against each other in changing coalitions, either "vertical" (zong 縱) or "horizontal" (heng 橫). Seven of them became dominant (the "seven dominating states" qixiong 七雄: Qin 秦, Qi 齊, Yan 燕, Chu 楚, Zhao 趙, Wei 魏, Han 韓), and among these, the kingdom of Qin prevailed, conquered the six other states and founded the "empire of Qin" 秦 (221-206 BCE).