A scenario represents an end user’s expectation of the behavior of the application under test. Scenarios in Twist are requirement specifications that also behave like executable acceptance tests. A scenario may be either manual and/or automated.
A well written scenario focuses on the test intent (what is to be achieved) as opposed to the test mechanics (how to achieve it). It is therefore important to write scenarios without coupling them to the user interface of the application under test. A good way to express the intent would be to describe the requirements or goals of the test, rather than expressing, say, UI level interactions.
If you are familiar with using user-stories for development, then a user story would typically be associated with one or more Twist scenarios. Any logical piece of functionality can be associated with one or more scenarios.