Sources used in the early days of film for screenplay adaptations were mainly novels and short stories. Today, they also include plays, non-fiction books, comic books and graphic novels, short films and even older film and TV scripts for remakes.
Writers and producers interested in adapting a piece of source material must first secure the film rights, usually for a fee, from the author and/or publisher of that source, if any copyrighted material is used, before proceeding with the project.
Adapting someone else's (or your own) work to a different medium can be a daunting task depending on the source material. Expansive novels may need to be stripped down greatly while still maintaining the central themes--with the opposite true for short stories--to be shaped into a project that will work in its new medium.
Some of the best and most successful films of all time have been adaptations of other works, including "Gone With the Wind," "The Maltese Falcon," "The Godfather," the "Harry Potter" series, and the 2009 Best Picture winner, "Slumdog Millionaire."
Many Broadway plays are also adapted from other sources, including movies, operas, novels, songs (or a collection of songs) or even other plays, as when a drama is converted into a musical.