A sequence diagram is a visual representation used in software engineering to illustrate how different components of a system interact over time. It shows the order of messages exchanged between objects, such as users, servers, or databases, during a specific process. Each object is represented by a vertical line, and the interactions are depicted as horizontal arrows, making it easy to understand the flow of information. These diagrams help developers and stakeholders grasp complex processes by breaking them down into simple steps. By visualizing the sequence of events, teams can identify potential issues, improve communication, and ensure that all parts of the system work together effectively.