What is concurrent users and simultaneous users ?

For example, axis bank application has below transactions
                           1. Login
                           2. Account summary
 ---------------------------Transfer money Action ---------
                           3. Click Transfer funds
                           4. Select "Beneficiary"
                           5. Enter "Amount"
                           6. Transfer
                           7. Enter "User id" and  Password and click "Confirm"
--------------------------------------Transfer Money Action end--------
                           8.Logout

So we have 8 transaction for this application and we have one Action (Transfer Money is one action but in that action you can have 5 transaction).

Concurrency : Performing same activity at a time by all users.Mean 100 users are login into the application at a time. But simultaneous users are login into the application one by one. Concurrency has various levels
          1.Transaction concurrency 
          2.Business Process Concurrency 
          3.Application Concurrency 


Transaction concurrency :
We need to test the application with 100 concurrent users, Means "All users(100 users) should perform the same transaction (login or   Account summary .... or  Logout) at a time" this is called transaction level concurrency.

Business Process Concurrency:

We need to test the application with 100 concurrent users for Transfer money Business action, means  "All users(100 users) should perform the same Transfer money action (here may be 30 users are clicking "transfer funds, 20 users are on selecting "Beneficiary", 20 users are in enter "Amount" page,10 users are in "Transfer" page  and 20 users are Enter "User id" and  Password and click "Confirm" ). 

Overall 100 users should be in transfer money action.


Application Concurrency :
All 100 users should perform different transaction or same transaction but at any time 100 user should be accessing the application.


Simultaneous users: All 100 users are login into the application one by one.