Good Samaritan laws protect caregivers from prosecution for medical mistakes, as long as the caregivers are acting in a voluntary manner without any expectation of reward (like the good Samaritan from the Bible story). Caregivers can't make gross medical errors, but instead are protected from liability for "reasonable" mistakes. Of course, defining "reasonable" is difficult. Victims tend to believe all medical mistakes are unreasonable.
Good Samaritan laws were initially intended to protect physicians and others with medical training. Over time, court decisions and legislative changes have helped some laws evolve to include untrained rescuers who render aid. There are several versions of good Samaritan laws.
Some laws still protect only medically trained rescuers, while others offer protection to any good Samaritan. The Code of Alabama (Section 6-5-332) restricts protection to trained rescuers or employees of the public education system, unless the victim is suffering a cardiac arrest. That means any victim of a motor vehicle accident in the state of Alabama, who isn't already dead, won't be helped by a lay rescuer unless that lay rescuer wants to take a chance on liability for any mistakes.
Not all good Samaritan laws limit protection of untrained rescuers. Vermont provides immunity from liability specifically for untrained rescuers, as long as they don't screw up really badly (defined as gross negligence). However, Vermont is unique in another way. All other states provide immunity if a person chooses to help, but do not require us to aid our fellow humans. Vermont does, however -- the state's good Samaritan law actually orders citizens to help those in need.
The role of the courts in shaping good Samaritan laws is a double-edged sword. Court decisions can nudge the interpretation of a law toward more protection for citizens, but the court can also make protections more narrow. A court ruling on California's good Samaritan law interpreted the law as intended only for "medical care," and created doubt about the law's ability to protect everyone who helps at an accident scene. Until this change, California's good Samaritan law provided pretty good coverage for trained and untrained helpers alike.
There's not enough room here to go over each state's good Samaritan law. It pays to check with your own state's regulations or laws to see how they differ and determine how much protection you really get. Just because a state doesn't provide immunity from liability, doesn't mean any mistake by the rescuer will automatically result in paying money to the victim. It just means there's no get-out-of-court-free card.
Keep in mind, there's never any liability for reporting an injury to the authorities and letting professionals take care of it.