The Precipice World separates the World of the Living from Soul Society.
Formerly used as a penal colony, the Precipice World is an alternate dimension that separates the World of the Living from Soul Society. It is made from layers upon layers of time spaces, which cause the the flow of time to be roughly 1/2000 of standard Earth time.
To prevent abuse of the time disparity, and formerly used as an execution method during its penal colony days, a large unstoppable being known as the Cleaner travels through the Precipice World and purges any beings caught in its path. Additionally, a dangerous substance known as Koryu lines the walls of the Precipice World and traps/pursues anything with spiritual pressure that it senses or comes in contact with.
Due to the Cleaner and other dangers present, the Precipice World usually serves as little more than a way of getting from Soul Society to the World of the Living and vice-versa for those who have no other path. However, the following are story events where events taking place there played a major role in the story of the series.
During Aizen Sousuke's assault on Soul Society, the Cleaner was destroyed, leaving the time disparity open for abuse. Isshin Kurosaki noted this while traveling through with his son Ichigo and aided his son in making use of it. This excess time and the training that took place during it allowed Ichigo to handily defeat Aizen during their next encounter.
Kageroza Inaba, a member of the information division of the Gotei 13 who was tasked with monitoring the Precipice World, is able to take control of hit during his attack on the Gotei. Kageroza alters the flow of time in the Precipice World and uses it as a way to trap the shinigami in whatever world they are currently in (Soul Society or the World of the Living).