Lawyers are called by the nature of what they do. So, when a lawyer limits their practice to a certain area of the law they get called that type of lawyer. A personal injury lawyer represents someone who has been injured in an accident because of what someone else has done. Someone was negligent or at fault and caused harm or injury to another person or persons. This is considered a "personal injury".
In most other areas of the law, lawyers charge hourly rates or a fixed price to do something but what is unique about personal injury attorneys is that they charge a percentage of the settlement or what has been recovered for the injured victim. What this means is that a client doesn't pay out of pocket expenses to their attorney, their fee is paid from the settlement check they receive.