Nobody knows for certain what happens to people when they die. This is because once people die, they can't communicate with the living to explain their experience. But sometimes, people are pronounced "dead", and somehow manage to return to life a short time later. Often, people who have such near-death experiences claim that during the time they were "dead", some strange things happened to them. Many people describe leaving their bodies and floating above them, seeing impossibly bright light, and viewing their entire lives pan out in seconds. Because they've essentially died and come back to life, they are able to recount their experiences to the living, and give us a short glimpse into the world of the dead.
Unfortunately, these claims are difficult to back up with solid evidence. Experiments to test out-of-body experiences have been widely discussed, but would be a challenge to carry out. This is because such experiments would involve inducing near-death experiences, which would be inhumane. One commonly discussed (but never performed) experiment involves placing a sign that reads, "You are dead" high up and out of viewing-range of the "dead" subject. If, when the subject regains consciousness, he or she knows what the sign said, then that person must have had an out-of body experience; otherwise, he or she wouldn't have been able to see the sign.