Living Wills are also known as Advance Medical Directives, there is no difference between the two documents.
A Living Will is a way for you to let your doctors and relatives know how you feel about certain aspects of medical treatment.
Of course in the unfortunate event that you do wind up in hospital you would normally be able to tell them yourself, but there are times when patients are not able to communicate, and doctors or relatives are called upon to make decisions for them.
A Living Will will leave them in no doubt about your wishes.
Your Living Will can include details about under what circumstances (if any!) you would like treatment to be discontinued. Just in case you don't pull through; you can specify what (if any) organs you would like to donate.
It is important to remember that a Living Will and your nominated Agent will only be consulted if you are not able to communicate for yourself.
You can make a Living Will here