For as long as people have been travelling from point A to point B, they've relied on maps to help them find their way. Workflow diagrams (also called flowcharts) fulfil a similar function, detailing every step that a process takes, including dependencies, responsibilities and tools. This allows users to logically plot out effective workflows, eliminate inefficient steps and blockers, collaborate among teams and see possible next steps at every stage of a project.
For an example of a workflow design tool that uses workflow diagrams, see ServiceNow Flow Designer. Flow Designer enables both professional developers and low-code business analysts to automate and integrate any business process as a digital workflow using natural language.
Activity diagrams take this concept further; an activity diagram is a Unified Modelling Language (UML) behaviour diagram that represents the workflow of activities within a system, using formally specified notation, symbols and rules. An Activity Diagram in UML is generally used in modelling software systems; they are considered behavioural diagrams, because they describe what must happen in the systems, they are modelling.