Hi Vetrivel,
There's plenty of ways how you can "federated" authentication with Azure AD, and thus "offload" the authentication elsewhere, for example an on-premises directory (which can be LDAP, or AD, etc...) --all of which is explained here:
https://support.office.com/en-us/article/Understanding-Office-365-identity-and-Azure-Active-Director...I suggest you start from that article, as it will provide you with lots of information on how authentication can be performed.
As far as "alternate ways" to authenticate AD users is concerned: there's plenty of choice with Azure AD. You can do cloud-based authentication, use Password Hash Sync, SSO, Pass-Through Authentication (PTA), or do Federated Auth (on-prem or with another cloud provider)....
Hope this helps!
-Michael
