With Sinatra you add in and with Rails you taketh away. You can use Sinatra for the same things that you use Rails for, but typical use cases are API's, since

With Sinatra you add in and with Rails you taketh away. You can use Sinatra for the same things that you use Rails for, but typical use cases are API's, since there's a lot of stuff in Rails that you don't want to use if your main output isn't html.