The paper provides a comparison of the basic DO (distributed object) concepts, namely "object", "instance" and "class" used in various standards of the DO world. Two families of standards are identified: those related to the modelling aspects (MOF and UML) and those related to the architectural and implementation aspects (CORBA and Java). Moreover, the RM-ODP standard is considered as it includes both aspects. The objective is to help out with a common understanding of these concepts. For this, we compare these concepts as defined in these standards according to the four layer architecture proposed by the MOF standard. An example is provided to illustrate the comparison.