OpenACS object types are the basic building block of the OpenACS psuedo object oriented datamodel. They are largely analogous to Classes in OO languages such as C++ and Java.
An object type typically has a set of static metadata and a set of object data for the actual instances.
Mainly found in the acs_object_types table and the acs_attributes table.
The instance data of a type is typically spread over several tables, each of which represents a level in the inheritance hierarchy. acs-object is the most fundamental supertype from which all types inherit.