Trails is a library that makes it easier for developers to build Twilio applications in the Rails framework by providing functional test helpers to simulate Twilio calls, allowing the application to recognize whether it's a Twilio call or a web request. Trails was built on top of the existing Twilio Ruby Library and has reusable controller and views for displaying HTML or TwiML depending on the source of the request. The library was created by Hemant Bhanoo as a solution to his own testing challenges while developing Twilio applications, and it's now available for contributors to add more functionality. Trails is well-received by its creator, who praises Twilio's developer-friendliness, and offers examples and resources for developers getting started with the library.