Ryan Singer:

What is at the core of an interface design? I think of the design not as a collection of screens or buttons or pixels, but as a collection of jobs that the user wants to do. In this article I want to give you a feeling for how to think of interfaces as made up of jobs, each with a beginning, middle and end.

I first read this in his site Felt Presence, now he repost it in Medium. Medium have been killing it lately, so many smart people started publishing their writing there.