About Twine
Twine is an open-source tool for telling interactive stories using hypertext. There are many story formats that work with Twine, each influecning how scripting in Twine works. As a relatively newer user of Twine, I started with Harlowe and found its features intuitive yet powerful enough for the types of stories I wanted to tell.
You can read more about Twine on its official wiki and Harlowe on its official documentation.