Twine is an open-source tool for telling interactive, nonlinear stories
Interactive, nonlinear stories == interactive fiction (IF) == text adventure games in the style of 1980s Infocom.It's interesting that the Twine website seems to actively avoid calling them IF or text adventures or games, and prefers "stories" instead. Maybe the community feels that those labels are too limiting. (I haven't played any contemporary IF or any Twine experiences, so just observing as an outsider.)