The directory stores objects. These objects represent real things such as users, groups of users, workstations, applications, data, distribution lists, etc. Each object has its own unique attributes. A user object will have attributes like full name, password, telephone number, and so on. The formal definition of all object types that can be stored in the directory is called the schema. The definition in the schema consists of a class definition for each type of object and an attribute definition for each attribute. The class definitions list the attributes that can be used to describe the class.
There are no set limits to the type of objects and their attributes that can be defined within a directory. The ISO has published a recommended set of attributes and objects in the X.520 and X.521 Recommendations. These are objects and attributes that the ISO membership has agreed provide useful information for a global "white pages" directory. There are international standards for syntaxes for select object attributes; phone numbers for example. Although some attributes have become standardized, others will never come to a global or national standard. This is perfectly reasonable; individual users have diverse needs and a directory service must accommodate a wide range of uses.