Basically it's just like AIR. You install a framework to launch applications. The Lark framework is composed of XULRunner, Ruby and some other stuff.
Then you download the Lark SDK to deploy your applications. If you already have a Rails application, you just need to package it and it's finish ! You can customize it with an application icon, a menu bar and more !
Creating an application is just creating a Rails web application. You can develop it with your browser and when it's finish, your application is ready to go.
Check out Lark