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 objects involved in the operation are listed from left to right according to when they take part in the message sequence.
Note That: Class and object diagrams are static model views. Interaction diagrams are dynamic. They describe how objects collaborate.
Here is a sequence diagram example for making a hotel reservation. The object initiating the sequence of messages is a Reservation window.