Yong Zheng was the 4th son of emperor Kangxi. Because Kangxi had many sons, the rivalry between each was very strong. Yong Zheng had to fight hard, and after the crown prince's failure to do his duties, Yong Zheng had a chance to become the crown prince. But because of strong rivalry, this decision was never made.
When emperor Kangxi was at his death bed, he called Yong Zheng over. It was rumoured that he poisoned the old emperor, and added a stroke to the proclaimation of emperor, that made the statement "...pass the throne to 14th son" to "pass the throne to 4th son". This rumour of course could never be proved.
After ascending the throne, he carried out a series of new policies that much benefitted the development of Qing dynasty. Just to name a few: he eestablished the "Junji Chu" (Cabinet of Military) and weakened the power of the princes to strengthen the central power; he improved the tax law by demanding taxes according to number of acres of land; by dispatching ministers to the minority areas, he strengthened the control of them. He was in power for only 13 years but he gave the foundation to the prosperous rein of Qianlong. Without his effort, Qianlong could not become the emperor he was.
However, his rein was full of turbulence as the other priinces were always trying to dethrone him. One time, in allaince with the 8 Kings, the 7th, 8th and 9th son of Kangxi almost dethroned Yong Zheng, if it were not for the 13th prince who came to help out. But because of this event, the 13th prince died and Yong Zheng lost the only brother that was close to him. Because this conspiration also involved Yong Zheng's first son, Yong Zheng, who did not want Qian Long to share the same tragedy as he, sent orders for the first son to commit suicide. This way, when Qian Long ascended the throne, he had no rivalries.
Although Yong Zheng achieved so much, because of his revival of the Wenzi Yu (supress of scholars and burning of books) and his cruelty, he had many bad names that survived till today. However, now, these bad names are slowly to be exchanged for the good things that Yong Zheng did.