To the best of my knowledge, when referring to internet DNS, you are referring to a much larger scope than Active Directory. Active Directory refers to a specific segment, usually a LAN-based segment of the internet and contains not only DNS information, but a lot more. Active Directory information can be used to supplement DNS, but primarily on a LAN or even a WAN that is separate and disctinct from the general internet.
Internet DNS provides relationships between IP addresses and "friendly names". DNS resolves the name:
www.somecompany.com to an IP address (or another "friendly name".
Active Directory's implementation of DNS allows you to lookup any organizational unit (OU) such as a workstation, a user, etc. and resolve not only IP addressing, but also a bevy of other resources.
Also, the protocols for accessing the two resources are slightly different.
You can access Active Directory information using LDAP. See this wikipedia link: http://en.wikipedia.org/wiki/LDAP
Hope this information helps,
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