Simultaneous Vs Active Vs concurrent users

1. Active User : The Use who has connection with resource like IIS or website.
1. Active User can be either Simultaneous user or concurrent users
2. Simultaneous user : Has an active connection with resource like DB or website not may not logged in at same time.
3. Concurrent Users are also active user who logged in at same point of time and race for system resources.
1. Concurrent users can have There are 3 different levels of concurrency.
1. Application level
2. Business process level
3. Transaction level

References 

1. http://www.geekinterview.com/question_details/42152
2. https://forums.oracle.com/forums/thread.jspa?messageID=1187614