What is Cisco Secure Access Control System (ACS)?

you can use ACS for two purposes either for controlling device access or network access.

Now a days we are using ACS 5.8.

You can perform these functions with ACS:-

1)Centralised Authentication for all user accessing any network device.

2)CustomiZe what a user can access.

3)Also save the information what task user have performed.(Accounting)