State diagrams express with a focus on states, rounded rectangles represent states, and the content written on the transition line corresponds to the activity in the activity diagram.

Activity diagrams express with a focus on activities, rounded rectangles represent activities, and content is usually not written on the transition line, except when making judgments, where the condition is written on the line.

A state refers to a condition of an object during its lifecycle, fulfilling certain conditions, executing certain activities, or waiting for certain events, represented by rounded rectangles.

Initial State: Represents the start of the state machine diagram, depicted with a solid circle, and there is only one initial state in a state machine diagram.

Final State: Represents the end of the state machine diagram, depicted with a solid circle with a ring, and a state machine diagram can have multiple final states.

Composite State: A composite state is a state that contains nested substates. Depending on whether the states exist simultaneously, they are divided into sequential substates and concurrent substates.

History State: A history state is a pseudo-state that indicates when the state transitions back to the composite state, it should be in the substate it was in when it last exited, rather than starting from the initial state of the composite state.

A transition is a relationship between two different states, indicating that the object will perform certain actions in the source state and enter the target state when a specific event occurs and a specific guard condition is met.

Triggering Event: The cause of the transition, which can be a signal, event, condition change, or time expression.

Guard Condition: A logical expression, the transition is activated when the triggering event occurs and the guard condition is true.

Action: Can be an assignment operation or arithmetic operation, or a sequence of actions, including sending a message to another object, invoking an operation, setting a return value, creating or destroying an object, etc.

Signal Event: Refers to an event where an object receives a signal, causing a state transition.

Time Event: Represents the passage of time, and an event may be triggered when a time condition is met, which can be relative or absolute.

Change Event: Refers to an event that occurs when a condition represented by a logical expression dependent on specific attribute values is met.

Call Event: Represents an object receiving a request for a call, and the object implements the operation through state transition rather than a specific process.

Controller: Such as elevator control systems

User Interface Flow: Such as login processes

Lifecycle Management: Such as orders, tasks, requests, etc.