A lawyer is a professional who is qualified to offer advice about the law or represent someone in legal matters. A lawyer can also be called an attorney, a solicitor, a counselor, a barrister, or — pejoratively — an ambulance chaser.
A lawyer can handle all sorts of legal matters from drafting wills to patent claims to defending people against criminal charges. Qualified lawyers have to attend law school and pass a bar exam in order to practice law. Lawyer, of course, means one who practices the law. "Law" itself comes from the Old Norse root word lag, which means something laid down or fixed.