What is the Rinzai School?

Quoted from Rinzai-Obaku zen (Link)
The Rinzai school emerged in China from the teaching line of Linji Yixuan 臨濟義玄 (J., Rinzai Gigen, d. 866), and was first introduced to Japan by the Japanese monk Myoan Eisai 明菴榮西(1141-1215). In Japan the Rinzai teachings underwent further refinement, especially in the lineage of Hakuin Ekaku 白隱慧鶴 (1686–1769), the great reviver of the Japanese Rinzai school. A second introduction of the Chinese Linji school took place in 1654, when the Chinese master Yinyuan Longqi 隱元隆琦 (Ingen Ryuki; 1592-1673) transmitted the teachings as they had developed in China from the thirteenth through the sixteenth centuries. Yinyuan’s lineage came to be called the Obaku school.
Throughout their histories these schools have sought, through the practices of zazen, koan training, and samu, to awaken the practicer to the Buddha-nature inherent in all beings. In the Record of Linji, Master Linji says, “On your lump of red flesh is a true man without rank who is always going in and out of the face of every one of you. Those who have not yet proved him, look, look!” The basic stance of the Rinzai and Obaku schools is the self-realization of this “true man without rank.” This self-realization, called kensho 見性, is regarded as the essence of Shakyamuni’s teaching, and has been passed from generation to generation in Zen, like, as the Zen expression has it, “water poured from one container to the next.”