The afterlife is the plane of existence where one's essence continues to exist after their death in the physical realm. It is governed by Yveltal.
The afterlife falls outside the domains of Dialga and Palkia, and as such, does not physically exist; nothing has any physical appearance or presence outside of what is perceived by its residents. The most common perception to those who have only recently entered the afterlife is an infinite field.
Time is neither consistent nor measurable, only relative, and based on the observance of given points being relative; that is, one event can happen before or after another event, but there could be no measurement of the amount of time between them.
The infinite expanse has gradually been shaped into a facsimile of life, including things such as towns and dungeons; though it remains nothing more than a perception, it may be used to combat apathy in the afterlife.
In general, passage into the afterlife is a one-way issue. Pokémon may enter without dying, but not without a great amount of difficulty and effort to maintain a presence there; outside of certain exceptions, only a Ghost-type Pokémon would be capable of it.
A deceased Pokémon may be able to influence the physical realm as a literal ghost, but only if they have not been guided to the afterlife. Once they have, they lose the ability to freely observe or influence the physical realm.
If someone wishes to be resurrected, they may go through a vague but difficult process to be reborn as a Ghost-type. Exception to this was granted during Giratina's attempted return; as the boundaries between the physical world and the afterlife had been weakened, zombies who had the cure administered were permitted a full resurrection, as were those who died during the onslaught.