Concurrency Testing (also commonly known as Multi User Testing) is used to know the effects of accessing the Application, Code Module or Database by different users at the same time.It helps in identifying and measuring the problems in Response time, levels of locking and deadlocking in the application.
Example
Load runner is widely used for this type of testing, Vugen (Virtual User Generator) is used to add the number of concurrent users and how the users need to be added like Gradual Ramp up or Spike Stepped.