From what I know a coma can be caused by many different things, such as serious head trauma, brain infections or drug and alcohol poisoning. Many people experience nothing when they enter into a coma, just like sleep with no dreams but others have reported that they can still hear what is going on around them. Usually waking up from a coma indicates that the person is getting better, and some people can apparantly make a quick full recovery, while others require years of physiotherapy.
As Oli said there are many reasons why someone may enter a coma from brain infections, head injuries or from unmanaged diabetes. The body goes into the coma as it has experience extreme trauma and can help to protect the brain from further injury.
In a coma, a patient is unconscious with minimal brain activity. Although the person is alive, they show no signs of being aware so it’s probably like being asleep but without dreams.
I’m not sure exactly why people wake up from a coma, but it probably happens when the body knows it is well enough to wake up again.
This isn’t really my speciality! The others have some very useful and more scientific answers. 🙂
As they said there are many different causes of coma’s, and it’s kind of like sleeping… I have had a general anaesthetic before, which is a type of drug-induced coma. I had it while I was in hospital having an operation… From my point of view it was just like going to sleep… and i do remember having dreams while it was going on! Then i slowly drifted in and out of consciousness after the operation whilst being sick, till i was completely awake 😛 I’m assuming I woke up because the drug had worn off!