If you mean the difference between NTLM and DCinterface please see the following document:
What is the difference between NTLM and DCINTERFACE authentication methods for Symantec Web Gateway? http://www.symantec.com/business/support/index?page=content&id=TECH97340
If you use NTLM or DCinterface you still must have the Symantec Web Gateway configured and setup for use with your LDAP source so various information such as group membership information can be polled from your AD.