Used especially for a female author; 'author' is more common today and gender-neutral. Mary Shelley is a famous historical authoress.
Pro Tip 2/3
Potentially Outdated
Some may consider 'authoress' old-fashioned; 'author' is preferred in contemporary contexts. She prefers to be called an author, not an authoress.
Pro Tip 3/3
Cultural Shift
Recent cultural shifts focus on gender equality, hence 'author' is frequently used for all genders. The award-winning authoress now identifies simply as an author.