Classes and Attributes

The world of Active Directory does not end right after understanding the classes and attributes there are several other components which need to be understood before saying that you really understand what Active Directory is.
In the implementation of directory service the designers had to take care to reduce down the complexities yet a brilliant performance. To achieve this they had to consider that the classes which are used to define an object with directory service need to be less in numbers.

For example, in a library card catalog, it would be a mistake to create a class called Somewhat-Less-Than-Riveting-Early-20th-Century-American-Novels, even though it seems like quite a few objects would fit in that class. In relation to the overall scope of a library, this classification would be too narrow. It would be better to have an attribute called Boring with a Boolean value. You could assign this attribute to the Book class so that objects derived from that class would get a Boring attribute that could be given a value of Yes or No or left empty. You could also assign the Boring attribute to the Periodical, Tape, and Video classes.

A directory can have hundreds of classes and many hundreds of attributes. If the attributes for each class had to be separately defined, the sheer number of perturbations would make the directory look less like a tree and more like an example of German expressionism.
Fortunately, attributes associated with a particular class often overlap those of other classes. For example, the attribute list for the Mailbox class includes all the attributes associated with the Mail-Recipient class with one addition, the Delivery-Mechanism attribute. So, instead of separately defining all the attributes in Mailbox class, LDAP allows the class to be defined as a child of the Mail-Recipient class. This permits it to inherit the attributes of its parent. The designer need only stipulate the new additional attribute or attributes that make the subordinate class unique.
Flow of the these attributes is like the flow of genes in the family tree. This figure (Seems like there is some problem with scripts running on the create post page so could not upload the relevant image, I will upload the same when the problem is fixed.) may help you to understand the flow.

Object Instances
Each object in Active Directory is derived from a specific object class. Another way of saying this is that an object represents an instance of a class. Each instance of an object class differs from another instance by having different values for its attributes.

Let’s assume that Merrick stands in front of a curious mob and exclaims,”I am not an elephant. I am a human being.” Had Mr. Merrick been a directory services designer, he could have clarified his point by adding, “I am an instance of the Human Being class, not the Elephant class. And the only difference between you and me is a relatively minor attribute of mine that has a different value from yours. So lay off, will you?”

Schema
A database schema defines the content and structure of the database. In a library card catalog, the schema would be a set of procedures and rules set down by the librarian.” Books go on green cards,” she tells you.” Videos go on red cards. File the cards alphabetically by Title in this cabinet and by Subject in that cabinet.” So on and so on. The schema for an LDAP directory service defines these items:

The attributes associated with each object class
The permissible object classes
The parent-child relationship of object classes, which in turn determines attribute inheritance
The data type associated with each attribute
The physical representation of the object in the user interface

Previous two posts were purely intended to help understanding LDAP information model. If you are really interested to understand the LDAP information model in depth then you can certainly have a look at RFC http://www.ietf.org/rfc/rfc2255.txt and then http://www.rfc-editor.org/rfc/rfc4512.txt , there are several actually which explain the standards for the LDAP information model.