This paper introduces a concept and associated descriptions to formally describe physical production resources for modular and reconfigurable production systems. These descriptions are source of formal information for (automatic) production system design and (re-) configuration. They can be further utilized during the system deployment and execution. The proposed concept and the underlying formal resource description model is composed of three different description levels, namely Abstract Resource Description (ARD), Resource Description (RD) and Resource Instance Description (RID), each having different scope and objectives. This paper discusses in details the content and differences between these description levels.