copy from "Windows 2000 Resource Kits"
Because of the existence of the class inheritance hierarchy in the schema, every object in Active Directory is in fact a member of many classes ? four or five on the average. For this reason, the objectClass index is prohibitively large (for example, 4n, where n is the number of objects in the system). In addition, objectClass has poor selectivity for many possible class values. For example, a search filter of (objectClass=securityPrincipal) returns every user and group object in the system.
On the other hand, objectCategory usually refers to the most specific class in the object's class hierarchy. Although objectClass can have multiple values, the attribute objectCategory has only one. Every Active Directory object has an objectCategory attribute whose value is a classSchema object.
Every classSchema object has an attribute called defaultObjectCategory, which is the object category of an instance of the class if none is specified by the user. For most classes, the defaultObjectCategory value is the class itself. In the search filter, you can specify objectCategory=X, where X is the ldapDisplayName of a class, and LDAP automatically expands the filter to objectCategory=. The objectCategory attribute has a syntax of distinguished name, and LDAP automatically converts the value for objectCategory to the distinguished name format. For example, if you use objectCategory=contact in the filter, the filter changes to objectCategory=cn=person,cn=schema,cn=configuration,dc= ("person" is the defaultObjectCategory for the class contact).