The Spiritual World is all around us right now as you read, its just on a different frequency level that has different vibrations. If the two worlds was built like a radio one would be on FM and the other on MW.
We as humans are spiritual beings who exist in the Spirit World as bodies of energy. When it is time for a soul to reincarnate on earth, a part of that energy or spark will go into the physical body and become a human being, whilst the rest of that energy stays in the Spirit World.
When the physical body stops functioning the "silver cord” connecting the physical body and the soul is severed. It usually takes place around one day after the physical death and can be closely related to how quickly a person is able to accept their death.
When a soul has had a materialistic view of life or have become emotionally attached to something they will find it difficult to move on and spend alot of time roaming around as a ghost. Souls that have caused distress will be taken to a lower level of the spiritual world were they will remain untill they acknowledge their wrong doings before they can move on.
Most souls will remain a short time here on earth after death but this can vary depending on their own mindset or consciousness. This is were spirit guides or past over relatives come in to assist the person into the next world. These souls have always been there to help in the event of an emergency or when you need them during difficult times.
Remember that the soul is multidimensional. For example, take the soul as a music album with around 25 or so songs stored on the disc. Currently I maybe track number 4 playing and this is my current life playing out. When the song or life is finshed, and I pass away, the song goes back to the album and another plays the next track or life. The album will keep the songs or memories of the life stored on its disc which can be accessed when needed and never fogotton. Mediums and clairevoyants can tap into this feild of energy to describe our past lifes or about our relatives. They can still talk to people using this enegry despite in some cases the soul in question has already began another life, hence why the soul is multidimensional.
So, the spark of life is never extinguished even when the physical body perishes and that we retain our individuality along with our memories which themsleves, along with consciouness, are non-material qualities. The spirit body continues to express itself then in non-physical form in a mental world rather than a material one. Thought action replaces physical action were life is more vivid and vital with unimaginable beauty and unbounded opportunity.
As mentioned before, everything that we know is made up of enegry and vibrations and these can vary occording to the environment we are in. In the physical world the vibrations are slow and dense, but in the spirit world vibration is at a much higher rate which is why the human eye can not see it since the frequency of the vibrations are too high. Take a dog whistle for example, we can not hear it due to the high pitch sound, but a dog can because they have a more sensitive hearing which can pick up that particular level of frequency.