Urine is a sterile liquid by-product of the body that is secreted by the kidneys. Though urine is not poisonous it is a waste product which contains unwanted compounds and can cause irritation to skin and eyes. It won’t cause much harm if anyone drinks it once or twice unintentionally or as an experiment but the intake must not be continued. However, it is possible to extract drinkable water from urine after proper processing such as the method followed by the International Space Station. For more: http://en.wikipedia.org/wiki/Urine#cite_note-6
You can drink your own urine although it is not generally recommended. If you consume meat products your urine would contain higher amounts of urea which can be toxic if consumed in large amounts. There are even some therapies which require drinking urine.
Urine is a sterile fluid, meaning it doesn't contain any harmful bacteria that might make you sick. If you drink the urine of a healthy person, most likely nothing will happen, but remember urinating is your body’s way of excreting waste that is not needed. While drinking your own urine may help you keep from becoming dehydrated if you are stranded without water, it’s not a good idea to drink it under regular circumstances.