text=An object is a model element that represents an instance of a class. While a class represents an abstraction of a concept or thing, an object represents an actual entity. An object has a well-defined boundary and is meaningful in the application. Objects have three characteristics: state, behavior, and identity. State is a condition in which the object may exist, and it usually changes over time. The state is implemented with a set of attributes. 
Behavior determines how an object responds to requests from other objects. Behavior is implemented by a set of operations. 
Identity makes every object unique. The unique identity lets you differentiate between multiple instances of a class if each has the same state. 
The behaviors of objects can be modeled in sequence and activity diagrams. In sequence diagrams, you can display how instances of different classes interact with each other to accomplish a task. In activity diagrams, you can show how one or more instances of an object changes states during an activity. 
There are three types if objects: Active, Passive, and Multiobject.