A specialist has an area of expertise in a profession or branch of learning. Call an ear, nose, and throat specialist if you can’t shake a stuffy nose, but go to a Shakespeare specialist to learn about King Lear.
Specialist started off as a word used just for doctors who focused particular diseases or parts of the body, but now a specialist can be someone with a narrow focus in any field. A specialist is any professional with a specialty — lawyers are specialists because each lawyer deals with a specific kind of legal work. To be a specialist you have to study or get experience in a specific field for a long time.