A lawyer is someone who argues for or against someone who has been accused of doing something bad. If they are arguing that they are innocent, they are a defence lawyer, and if they are arguing that they are guilty they are a prosecution lawyer. Lawyers specialise in all sorts of fields, like divorce, family law, constitutional law, intellectual property law and many other types of law.
Anyone can be a leader. Being a leader means you are the ruler of something or a great role model for others.
A librarian is someone who works in a library. Librarians deal with the storage and organization of information, traditionally in the form of books but today libraries store information in all sorts of digital formats, as well as collections of music and videos.