Active Directory is a database based system that provides authentication, directory, policy, and other services in a Windows environment
LDAP (Lightweight Directory Access Protocol) is an application protocol for querying and modifying items in directory service providers like Active Directory, which supports a form of LDAP.
Short answer: AD is a directory services database, and LDAP is one of the protocols you can use to talk to it.
Active directory is a directory service provider, where you can add new user to a directory, remove or modify, specify privilages, assign policy etc. Its just like a phone directory where every person have a unique contact number. Every thing in AD(Active Directory) are considered as Objects and every object is given a Unique ID.(similar to a unique contact number in a phone directory.
Ldap is a protocol specially designed for directory service providers. Windows server OS uses AD as a directory server, AIX which is a linux version of IBM uses Tivoli directory server. Both of them uses LDAP protocol for interacting with directory.
Apart from protocol there are LDAP servers, LDAP browsers too.
