I am not entirely sure, but I think that it is a document that appoints someone to make healthcare decisions for you if you are not able...
The problem with a doctor or even document making these decisions is that there are endless possibilities that you cannot anticipate. My elderly mother did not want feeding tubes etc... my brother was in charge - the doctor explained that w/a feeding tube for a week or two, she will probably survive and come back to normal... w/o - she will die... he made the decision to override and she did get to live on for several years...
maybe that is a living will - again, I am not sure but I did sign something that left a person in charge of those decisions for me..