Sequence diagrams
A sequence diagram is a Unified Modeling Language (UML) diagram that illustrates how a group of objects interact and operate with each other sequentially. The sequence diagram consists of a group of objects that are represented by lifelines and the messages that they exchange over time during the interaction.
Lifelines
Lifelines are objects that interact with each other in a sequence. For example, in an online shopping system, a product database, customer, purchase order, payment, and deliver systems can be represented as lifelines.The head of the lifeline is a box that consists of the unique name, and the type of the object such as class. A vertical dashed line that descends from the head of the lifeline is called the stem that represents the timeline for the instance of the object.
Messages
Messages describe the sequential interaction that happens between the instances of lifeline objects, such as start or running an operation or sending or receiving a signal. You can create new lifelines, create lifelines from existing elements, or assign element types to existing lifelines.