What happens to the soul after it leaves the body?
Death is when the soul is carried by the mind to its next body.
Let me tell you about what happens at the point of death, according to the Bhagavata Purana.
If you have been very good and pious, you will upon leaving the body be led to the planets of the demigods - the heavenly planets. There you will enjoy for millions of years before again you will be born on earth as a human.
On the heavenly planets you enjoy basically as you do on earth - sex, drugs and rock-n-roll - but it takes place on a subtle plane, the astral plane as some call it. You are in a subtle body - a psychic, finer physical body that doesn't exude excretions. IOW, you don't perspire, fart or stink.
In heaven there are heavenly dancing girls and the soma-drink - an intoxication that makes you feel good and prolongs life. The music is delivered by the gandharvas - some sort of angles who are the musicians of the heavenly planets. On the heavenly planets there is no disease or old age, but eventually you die and fall back to a human body on earth.
Now, at the point of death, if you have been very cruel and evil not caring or empathizing with other living entities, you will be picked up by the Yamadhutas, the messengers of Yamaraja - the god of Death - and taken to the hellish planets, where you will be tortured and undergo purgatory for what feels like eternity. Like on the heavenly planets, it happens on a subtle, astral plane, so a moment can feel like a long, long time - like in a dream. After having ended your prescribed suffering, you will again be born on earth, but not necessarily from a human womb. You can be born in any of the 8.400.000 species of life.
If you have just been an ordinary, somewhat decent human, neither too good nor too evil, like people are most, at the point of death you will fall into a deep sleep and stay like that for 7 months after which you'll wake up and find yourself tied up in a womb. The situation for the fetus is full of pain and distress. You are in full consciousness but you are totally bound up and gagged, it's like being bound up very tightly in a sack. If you are lucky, you will remember everything you did to land you in this particular womb, why you will be born to certain parents. Then after 2 months of such hell you are squeezed out of a hole much too little for you, and born in shock into total forgetfulness. The foster state is described in the Bhagavat Purana in great detail.
If, however, you have practiced spiritual life and think of Krishna at the time of death, you will return to the spiritual world in your original constitution of eternity, knowledge and bliss. Or you will be born in some universe on some planet where Krishna has descended to reveal His pastimes. When that period has ended, you will return with Krishna to Goloka either as one of His friends, His girl-friends or as His elder in a parental mood.
The choice is yours. Where ever you go after death will be according to how you formed your mental body. The mental or the psychic body is formed after the activities of your gross, physical body and the mentality you are cultivating.
And whoever, at the end of his life, quits his body remembering Me alone at once attains My nature. Of this there is no doubt. (Bg. 8.5)
Whatever state of being one remembers when he quits his body, O son of Kunti, that state he will attain without fail. (Bg 8.6)