When you finish the test the computer will give you the option of canceling the test or accepting it. If you accept the test, your score will be displayed. That score is your official GRE score and it will be sent to all schools that you designate. If you cancel, nothing happens but you will have to reschedule and take the GRE again. Note that you cannot take the GRE more than once in a month.
Most students are unsure of their performance, so the option to cancel is attractive. In most cases, you should fight the urge to cancel your score because that hunch is likely just a function of nerves. Sometimes, however, canceling might be a good idea, such as if you are ill, suffered a particularly stressful experience near the exam (like an accident on the way to the testing center), or some other emergency interfered with your performance.