Please tell me who in case of tickets will be notified if on mail events I check "FOLLOWERS" ?

All users who "suscribed" to the ticket (see icon next to email on ticket detail) and all those you subscribed to the project of the ticket (it is new feature on V6.5, subscriber to the project are automatically subscribers to all items of the project)

On ticket we have also field "ACCOUNTABLE" - how to make notification to this resource ?

Select accountable on email definition.
NB : after activating Accountable management in global parameters, you may have to disconnect to see this new option when sending emails.

If I change GLOBAL parameters and set:

What will be the consequence compare to have this option set to "NO" ?

Accountable will be visible on Tickets.
Accountable will be selectable as recipient of emails and alerts.