For each kind of program element there is a corresponding basic metaobject class. These are the classes: class, slot-definition, generic-function, method, and method-combination. A metaobject class is a subclass of exactly one of these classes. The results are undefined if an attempt is made to define a class that is a subclass of more than one basic metaobject class. A metaobject is an instance of a metaobject class.

Each metaobject represents one program element. Associated with each metaobject is the information required to serve its role. This includes information that might be provided directly in a user interface macro such as defclass or defmethod. It also includes information computed indirectly from other metaobjects such as that computed from class inheritance or the full set of methods associated with a generic function.

Much of the information associated with a metaobject is in the form of connections to other metaobjects. This interconnection means that the role of a metaobject is always based on that of other metaobjects. As an introduction to this interconnected structure, this section presents a partial enumeration of the kinds of information associated with each kind of metaobject. More detailed information is presented later.