Generally in the advertising world, someone who writes words, slogans, headlines or copy for an ad (No matter which type: Print, internet etc.) is called a Copywriter. The copywriter gets the information about the target audience (who you are selling to), and information about the product or service itself in order to write copy/text that will appeal to the viewers.
Programmer, Computer Programmer, or Software Engineer depending upon whom you ask.
Rita is the character in Jane Eyre to whom Jane writes to in the beginning of the novel. She is Jane's friend from Lowood school who becomes a governess for a family in Ireland. Jane writes to update her about her life at Thornfield Hall.
The person who makes a will (or 'testament') is a testator.
A composer is a person who writes music.
A person who writes laws is called a legislator or a lawmaker.
The person who writes a check is called the payor. Or in the case of my brother, a sucker, when he writes another check for his girlfriend.
A Philosopher
An autobiographer is a person who writes the story of his or her own life.
A person who studies or writes history is called a historian.
A person who writes the libretto for an opera is simply referred to as the librettist.
testator
A person who writes plays is known as a playwright. An example of a playwright is William Shakespeare.