Views present structural information about the system. However, structural information is not sufficient to allow reasoning about some system properties. Reasoning about deadlock, for example, depends on understanding the sequence of interactions among the elements, and structural information alone does not present this sequencing information. Behavior descriptions add information that reveals the ordering of interactions among the elements, opportunities for concurrency, and time dependencies of interactions (at a specific time or after a period of time).
Behavior can be documented either about an element or about an ensemble of elements working in concert. Exactly what to model will depend on the type of system being designed. For example, if it is a real-time embedded system, you will need to say a lot about timing properties and the time of events. In a banking system, the sequence of events (e.g., atomic transactions and rollback procedures) is more important than the actual time of events being considered. Different modeling techniques and notations are used depending on the type of analysis to be performed. In UML, sequence diagrams and statecharts are examples of behavioral descriptions. These notations are widely used.
Statecharts are a formalism developed in the 1980s for describing reactive systems. They add a number of useful extensions to traditional state diagrams such as nesting of state and "and" states, which provide the expressive power to model abstraction and concurrency. Statecharts allow reasoning about the totality of the system. All of the states are assumed to be represented and the analysis techniques are general with respect to the system. That is, it is possible to answer a question such as Will the response time to this stimulus always be less than 0.5 seconds?
A sequence diagram documents a sequence of stimuli exchanges. It presents a collaboration in terms of component instances and their interactions and shows the interaction arranged in time sequence. The vertical dimension represents time and the horizontal dimension represents different components. Sequence diagrams allow reasoning based on a particular usage scenario. They show how the system reacts to a particular stimulus and represent a choice of paths through the system. They make it possible to answer a question such as What parallel activities occur when the system is responding to these specific stimuli under these specific conditions?