Flows are just as important to good interfaces as individual screens are. Specific sequences of actions lead customers … as they try to accomplish their tasks. But … flows are hard to communicate during the design process. Drawing out every state of a flow is too time-consuming … instantly outdated as screens change. Flows written down into stories or paragraphs are hard to reference and they don't easily decompose into checklists for design and review.

Flows are made out of individual interactions. A screen offers some possibilities and the user chooses one. Then something happens, and the screen changes. 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.

via Signal vs. Noise.