FAST is a Focused Assessment with Sonography for Trauma that utilizes bedside ultrasound to rapidly identify intraperitoneal pericardial hemorrhage in trauma patients.
It is indicated in abdominal trauma patients who are hypotensive and/or who are unable to provide a reliable history. It is an adjunct to the primary survey (ABCs). In adults a positive FAST (blood in the abdomen/pericardium) means a trip to the OR for laparotomy. In children small amount of intraperitoneal blood is not as significant as in adults, and many can be managed conservatively.
Here are the locations for FAST
Images courtesy of Trauma.org - This is a great overview of FAST - Check it out here