Simultaneous logins: are number users logging in at the exact same time.

EX:100 users pressing the logging button at 9:00am

Concurrent logins: are number of users currently logged in to application as any given time

Total users with accounts:  1000

Concurrent number of users: 100,  at any given time, there are 100 users logged in and they are at various different steps in the business processes of the Application.