What do you meant by active and passive objects?

• Passive Object
  
  
  • It is activated when it receives a message from other object ( work when being hit)
    
    
• Active Object
  
  
  • It can change its state by itself and send a message to other object if necessary
    

Active objects are one which instigate an interaction which owns a thread and they are responsible for handling control to other objects. In simple words it can be referred as client.
Passive objects are one, which passively waits for the message to be processed. It waits for another object that requires its services. In simple words it can be referred as server.