What are the differences between LDAP and Active Directory authentication?

I'm not sure you really know the difference between LDAP and Active Directory. Active Directory is built on top of LDAP, just like HTTP is built on top of TCP. You can't make any meaningful comparisons.

Saying LDAP authenticates is like saying the HTTPS charges your credit card when you're buying something online. The LDAP protocol provides the information to the directory server which does the authentication, just like HTTPS provides your information to the processing server that does the processing.

Active Directory allows you to have multiple objects in a domain with the same CN, as long as they don't have the same parent. What it restricts is multiple objects with the same sAMAccountName attribute in the same domain.

Maybe you are interested in the features that Active Directory has that other Directory Services don't? Or how Active Directory uses LDAP to perform higher level authentication requests like username bob?

Active Directory is a product and a directory service. LDAP is an application layer protocol and an IETF standard. Active Directory supports and implements LDAP, therefore allowing RFC-compliant LDAP clients to talk to it and perform LDAP operations such as bind, search, etc.