A user flow is the designed path a user takes to reach a goal in a product – the sequence of screens, decisions and actions, e.g. from cart to order confirmation.
Designing flows forces answers to questions easy to miss when designing single screens: where did the user come from? What must they know to continue? What happens when they make a mistake or go back?
A flow is drawn as a diagram: screen rectangles, decision diamonds, transition arrows – including alternative paths and errors. Such a diagram cheaply exposes problems: loops with no exit, redundant steps, missing states.
Practical rule: every step in a flow is a place where some users drop off. Shortening critical paths (purchase, registration, contact) is one of the most effective forms of conversion optimisation.