For people who wish to know more about the afterlife in Buddhism.
Buddhism has a best heaven. Everything is just the way you want it to be. In this heaven, there is no reason to change anything. You are ultimately happy. The problem is that it's not permanent, as is everything in Buddhism. One day in the heaven realm is equal to 400 human years, and your stay is four thousand heaven years, so you will be there a really long time.
But, one day the karma that put you in this heaven will be used up. You are only in heaven as long as your Karma account has merit in it. You can only draw from your Karma account while in heaven, because there in no way to make a deposit. You can't practice generosity or compassion, and you're not striving to gain wisdom. When the karma that put you in heaven is used up... you will be reborn.
The second heaven realm, which is a lower one, is where things are almost perfect. I call this the Donald Trump heaven. It could be better, if only you owned one more building or house. You see, there is still some desire associated with this heaven realm, and so it can't ever be perfect.
The next realm is the human realm, where all of us find ourselves in this lifetime. This is the best place for us to be, because this is the only place we can become enlightened. We cannot become enlightened in heaven, things are too nice, and we have no reason to strive. We cannot become enlightened in hell, because things are so bad, all we do is suffer.
In this human rebirth, we have enough happiness and joy to keep us from taking our own lives, and we experience anxiety and fear to keep us striving. We cannot relax too long in any one mental state as a human, because all things are in a constant state of flux.
The next lower realm, is the animal realm. The animal realm is marked by wanting to have sex, wanting to have food, wanting to have sleep, and being totally confused. Those are the four characteristics found in the animal realm. So you can see, we are not likely to become enlightened as an animal.
A Zen question-- Does a dog have Buddha nature?-- comes to mind. Yes, a dog does have the potential to become enlightened, but only in the human realm.
Can animals be reborn as humans beings? Yes, if they come into contact with the Dharma, see a Buddhist temple, or smell incense burning. The contact can plant a Dharma seed which takes root when they're reborn as humans. They can achieve their full potential and become enlightened, but only as a human being. So, it's up to all of us to help our pets be reborn in the human realm.
The next realm is called the hungry ghost realm. The hungry ghost is often pictured as a giant creature, with a large stomach and a pinhole for a mouth. It can never end it's hunger no matter how much it eats, it never finds satisfaction.
In the hell realm, the worst place, you find the most suffering. Your are given little hell bodies when you enter. Then, one day you might be walking through a forest, when all the leaves on a tree turn into razor blades and fall, cutting you into a million pieces. You cry out in pain, and your hell body resurrects, so you can be killed over and over again.
The only way to get out of the hell realm is to burn through the karma that put you there. Suffering is the only act of purification in hell, and much suffering is necessary before the next rebirth.