When most people talk about a person being on life support, they're usually talking about a ventilator, which is a machine that helps someone breathe. A ventilator (or respirator) keeps oxygen flowing throughout the body by pushing air into the lungs. It's used temporarily for conditions like pneumonia, but it may be needed longer for someone with lung failure.
One end of a tube goes into the windpipe through the nose or mouth. The other end attaches to the electric pump. Some people get medicine to make them more comfortable and sleepy while on a breathing machine.
When a person's heart stops, doctors will try to restart it. These life support methods include CPR, which keeps blood and oxygen flowing throughout the body, electric shocks (called defibrillation) to get the heart beating again, and medication to help the heart work.