Flows are made out of individual interactions. A screen offers some possibilities and the user chooses one. Then something happens, and the screen changes. It’s an ongoing conversation. Each moment in a flow is like a coin with two sides. The screen is showing something on one side, and the user is reacting on the other side. My flow diagrams illustrate this two-sided nature with a bar. Above the bar is what the user sees. Below the bar is what they do. An arrow connects the user’s action to a new screen with yet another action.
Here’s a simple and concrete example. To add a to-do item in Basecamp, first you go to a list. Then you click to “Add an item.” The form appears. You fill in the item content and submit the form, and if your submission is valid, the item appears and flashes yellow. Here’s a shorthand version of this flow:
(Link: A shorthand for designing UI flows – (37signals))