It's a gigantic field. There are differences between the Hindu and the Buddhist take on the subject. As far as I know, Hindus believe that once you've started incarnating as a human, you can't reincarnate as an animal. Buddhists believe that if you augment your ignorance in any way you will indeed transmigrate to the animal realm because the main psychic poison that defines the animal realm is ignorance.
Too much hate will condition your mind-stream to the hell realms. In effect, a mind full of hate is a kind of hell realm unto itself anyway, even though the mind might currently reside in a human body. Once the causes and conditions of your human rebirth are spent, you will transmigrate to a hell realm and remain there until all of your "hell realm karma" is spent.
In Buddhist belief, you can even transmigrate to the god realm - if your karma is good enough, but it's a dangerous thing to do because generally speaking, the gods consume merit but don't create any. Their lives are so fabulously god-like that they don't experience suffering, therefore they do not learn empathy in that life and so they don't engage in compassionate activity that would generate even more merit. When the positive karma that enabled them to incarnate as gods is all spent, then, any negative karma carried over from other lives in an even more distant past that they haven't purified, will cause them to fall to a hell realm and then, they have to work their way up again, through the hungry ghost and animal realms until they become human again.
Karma, in Buddhist thought can be defined as "volitional momentum". It is not administered by any external agency. There is no God who judges you and sends you here or there. There are no case managers who call you in for a meeting and discuss your good and bad points and offer you a nice, therapeutic incarnation which will not involve too much suffering but will enable you to purify the karma you need to purify. Your karma is a kind of momentum built up by the accumulation of activities of body, speech and mind.
Think of a starship travelling through space. To the left is a black hole. To the right is a wormhole that will take you to the Alpha Quadrant.
Negative karma creates a leftward tending trajectory. Positive karma creates a rightward tending trajectory. Using this metaphor, you will see that a God, or cosmic judge is not needed to swat the ship into the black hole. Whether the ship falls in or not, depends on the course it was already travelling in, combined with whatever capacity it has to manouevre at the time.
Another element of Buddhist belief regarding transmigration, at least in the Vajrayana schools is that when you are in the state between death and rebirth, various Buddhas and Boddhisattvas will in fact manifest to you in the between state and invite you to liberation. The problem is, if you don't have the karma to recognize them, you will be unable to take up the offer.
Lights will appear at various stages. Each time, there will be a light which is hard and bright and intimidating because you feel that you can't hide anything about yourself. It exposes too much. Another light will appear which is soft and smoky and much more comfortable. Do not go into the light which is soft and smoky and much more comfortable. Go into the light which is hard, bright and intimidating. The soft, smoky lights are a kind of trap, not set by any evil agency trying to recruit souls for its cute little hell project, they arise in response to your karma.
Basically, the cosmos doesn't care where you go. It merely automatically provides the opportunities that accord with your karma. Opportunities for bliss, for learning or for torment and it all happens automatically according to natural law.
This is not the last word on Buddhist belief regarding transmigration and karma, this is merely my own limited understanding. I am the worst Buddhist I know, so take what I write with at least a heaped tablespoon of good quality salt.
