This is a reference guide for the Twine editor. It describes Twine's user interface and introduces the larger ecosystem that surrounds it. If you've never built anything with Twine before, this will guide through the basics and point you to resources that will help continue to learn.
You don't need any previous programming or game-making experience to be successful with Twine. These elements can be gradually introduced into the things you make as you grow more comfortable with the application.
Text-based storytelling. Twine is very text-focused, though you can use images, sound, and video.
Branching narratives. Twine's user interface is designed to make it easy to visualize the flow through branches of a narrative.
Web-based publishing. Twine publishes to HTML files which can be uploaded on any web hosting service or shared privately. People can play the things you make without installing any extra software.
This is especially difficult to define, because authors are so ingenious and constantly push at the boundaries of what is possible. Because Twine is deeply enmeshed with the web platform, anything that can be done in a browser can be done with Twine--but you might be better-served using a different tool.
Heavy use of multimedia. It's possible to incorporate images, sound, and video into the things you create with Twine, but the workflow can feel awkward, especially if you are using lots of multimedia assets. You might be happier using a tool like Ren'Py.
Online and multiplayer play. There have been experiments with making Twine games that are playable by multiple people simultaneously, but doing so requires a good deal of programming knowledge. There have not been many modern successors to the MUD model, but Seltani is one.
See Limitations for more details on some of the above and some possible ways to work around these issues.
In order to use Twine most effectively, you should spend some time reading the documentation for the story format you're using. (Wait, what's a story format?)
You might also find the Twine Cookbook useful to read. It contains example code and explanations for things Twine authors often want to do.