At its heart, Twine is a tool for creating hypertext. The difference between hypertext and a linear story, the kind found in books and magazines, is that it allows the reader to have some measure of agency. In other words, the reader has some ability over what he or she reads next. In a story about a haunted house, for example, the reader might be able to tell the protagonist to Turn around and run or Venture deeper into the mausoleum. In a nonfiction piece, the reader might ask to learn more about my aunt who went missing. The convention that has emerged over the past three decades is that readers navigate hypertexts by clicking links. In this sense, you're already a seasoned hypertext reader. You clicked several links to reach this text, after all, and you've probably clicked an uncountable number of links in your life so far.
Because hypertext branches so much, it's easy to get lost in your own work. Much of Twine is dedicated to helping you keep track of your work's structure visually with a story map, so you can see what your readers' experience will be like.
Can you build games with Twine? Of course! Twine has the capability to do conditional logic, so if the protagonist finds a key in an early part of the story, he or she can use it to open a door later on. It can also incorporate variables, which encompass the traditional trappings of games such as hit points and score. These, along with agency, are foundational concepts of interactivity, the currency of game design.