What is a Realm?
Realms are like ships out at sea. The sea is purgatory. When you die, you fall into purgatory and await the next ship to pass by (aka: reincarnation)
Realms of Life
Realms of Death
An entire universe/dimension. A single reality of that universe. Inside a Realm is an existence with many planets/worlds and lands, lush with various forms of life (or no sentient creatures at all, but life exists all the same).
Within a single realm, are different points in time. It is the same realm, but depending on when you enter that world, the time era will be at a different stage. It could be an entirely different 'reality' or a different time period of that exact same Realm. All realms float within the waters of Purgatory (where souls go to float after they die).