Author
Basics:
Salary
The amount you earn depends on what work you're doing, and is undefined.
The route of traditional publishing starts out with an advance. Let's say a publisher would like to sell your book. They choose the amount of money they'd like to pay for it- this depends on how popular they think your book will be- which could either be very low or very huge. JK Rowling was only payed 4,000 for or Harry Potter series, originally. Stephanie Meyer- the author of the Twilight series- was payed about 750,000 dollars for her advance, because the publisher knew it was going to be smaller.
Once the advance is paid, you begin selling your books. Before you can make any additional money off of this, you have to earn back your advance. JK Rowling had it easy, and earned her 4,000 dollar advance in no time. However, if the publisher thought your book would be more popular than it is, you'd have a very hard time meeting that payment, and might not even make any additional money.
Authors are also payed in royalties- which means they earn very little money off of each book they sell. If a book is sold at 20 dollars, the author might only get 2.
To become successful as an author, you've got to write a lot of books.