Heavens and hells are certainly a part of traditional Buddhist world-view, but the meaning is very different from the Western concepts:
1. There's no "Judge" who condemns you to either desitnation - instead they are just rational effects of your actions (physical and especially mental, including thoughts).
2. They are not eternal - everyone will be reborn, until you escape the round of births and deaths, which is only done through enlightenment.
3. It's fully possible to interpret heaven and hell as symbolical descriptions for either states of mind or life-situations caused by earlier actions (karma). For example, heaven can be the life of an exceedingly rich person, having the feeling he has power over his world (business imperium, lovers, servants, and so on), forgetting everything is impermanent, forgetting he will meet sickness, old age and death, etc. Life as an animal could be a person living like a vegetable, just watching TV, playing computer games, drinking beer, having sex, in short, not caring about the results of your actions and not caring about others. A hell life could be the experience of a mentally sick person (depression or anxiety neurosis, for example), experience of war, torture or concentration camps, and things like that. Suffering so great that you don't even have space in your mind to bother about mental training, compassion and plans for your future - all your thoughts are about surviving or getting our from your hell.
Like in traditional Buddhist cosmology, also in this symbolic/psychological interpretation, much more people dwell in hell and animal realms than in heaven. If you look around in the world, there's so much suffering caused by war, torture, hate, craving, prejudice, oppression, and so on.
Actually, I would say it's not important whether you interpret it traditionally or symbolically. The effects and motivation for your practice will be basically the same. Your actions have effects. Try to make more actions that have positive effects, try to avoid actions that have negative effects. Whether your bad actions lead to a life in a "hot hell" or in Nazi gas chamber is not important in this regard - you want to avoid it nevertheless. Just choose the interpretation that makes sense for you. If it doesn't make sense for you it isn't useful for you. For most Westerners, the symbolic/psychological interpretation is much easier to accept. You can start with this, and think again when you have studied and practised more deeply. Anyway, they can both be true at the same time.