When we speak about different entities from different realms we are speaking about three realms in particular - Earthen, Spiritual & Astral.
Earthen :: The planet Earth. This is the realm our physical bodies reside within. There are literally hundreds of thousands of races of entities who have been born and become extinct on this realm. Thousands of mystical entities exist within this realm alone and are capable of peacefully coexisting with humans. This realm is a grounding area useful for humans to make a connection with Mother Nature and the Earth's core powers.
It is a birthplace of mystical knowledge, a gateway to both the Spiritual and Astral realms and represents a median point for human existence. Its fragility is also its strengthen lending an everchanging atmosphere for the entities residing within it to grow and adapt. It possesses alone extraordinary powers giving those who seek them a vast influence and giving everyone on the planet a taste of its capabilities through its natural disasters.
Just as the other realms this one holds an unimaginable trove of secrets from its ocean floor, under and above water caves, soil, sand, lava... and swirling through its atmosphere the spiritual stains left by billions of entities since the beginning of time. With each century bringing a more dense air clouded with the spiritual energy of those billions passed.
Spiritual :: The realm that serves as an originating point and a gateway to other realms. All entities who exist on Earth will pass through the Spiritual realm at death on their way to the afterlife. There are many entities who find life starts within the spiritual realm, most of the races are guardian races who serve both Astral entities and Earthen entities.
The Spiritual realm is buzzing with energy and can be both a torrential source of power and a tender comfort. It is the gateway for humans to make connections with humans who have passed. Those who wish to exert medium powers will find the Spiritual realm is their platform for communication.
It exists within and around humans allowing any who are open to making a connection a swift retrieval of spiritual energy. Humans are capable of existing within the Earthen and Spiritual realm simultaneously when in the proper state of mind. On a basic level humans who practice religion find themselves often within both realms such as in times of prayer, meditation, traditional ceremonies, etc. On a more intense level those who practice Spiritualism can walk both planes at the same time maximizing their powers of greater influence with entities and spirits in both realms.
Astral :: The realm of the heavens. All Immortal entities exist within this realm as well as djinn and angels amongst others. It is a realm of tremendous mystery as it holds the most power and possibilities. The Astral realm is a beacon of pure light and a strong source of a compelling energies.
The energies of the Astral realm are ethereal and can leave a lasting impression on a human when they first experience what the realm has to offer. Some have walked through one of the astral portals that exist on Earth's realm and have described it as being wrapped by a warm blanket, the pure feeling of happiness, and a tingling sensation at the base of the neck, spine or throat. Astral portals are a natural occurence on Earth and though the chance a human will accidentally walk into one is one in a million those who are lucky enough never forget. The portals do not transport humans but it is rather a gathering of energy that comes and goes.
Astral entities can walk all three realms at the same time at their discretion. They are gifted this ability and therefore can exact great influence over entities of all the realms. They are capable of providing life-altering magic to humans and spirits. The Astral realm is the pinnacle of mystic connection and power.