A sequence diagram is an interaction diagram that details how operations are carried out -- what messages are sent and when. Sequence diagrams are organized according to time. The time progresses as you go down the page. The participants (lifelines) involved in the operation are listed from left to right according to when they take part in the message sequence. The sequence diagram example here is a sequence diagram for adding a course. The lifeline initiating the sequence of messages is :Registrar
1. The :Registrar sends a addCourse("UML) message to a :RegistrationManager. The :RegistrationManager then sends a create() message to the Course class.
2. Each vertical dotted line (tail) represents the time that a lifeline exists.
3. Each arrow is a message call.
4. Activations indicate when lifeline has focus of control.
5. The activation bar represents the duration of execution of the message.