A sequence diagram is a type of interaction diagram that illustrates the interactions between objects or components in a system over time. It is used to model the behavior of a system in terms of the messages exchanged between objects or components.
Typical elements:
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Objects: An object represents an instance of a class or a component in a system. Objects are shown as rectangles with the name of the object at the top.
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Lifelines: A lifeline represents the lifespan of an object or a component in a system. Lifelines are represented as vertical lines that extend from the top of an object rectangle.
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Messages: A message represents a communication between objects or components in a system. Messages are represented as arrows between the lifelines of the objects or components. They can be synchronous or asynchronous, and they can have parameters and return values.
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Activation bars: An activation bar represents the period during which an object or a component is active in processing a message. Activation bars are shown as horizontal bars on a lifeline.
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Combined fragment: A combined fragment is used to group messages or to specify conditions or loops in a sequence diagram. Combined fragments are represented as rectangles with a specific notation that indicates the type of fragment.