CAD PDM, PLM vs ERP*

PDM and PLM: does not mean "CAD software". PLM and PDM describe database systems for design, manufacturing, change and workflow management with which CAD software integrates. Essentially PDM (Product DATA Management) organizes product design and change data according to a standardized schema of relations between documents and projects, usually inside a single organization. PLM (Product LIFECYCLE Management) loosely facilitates larger-scale workflow collaboration between multiple organizations and external partners, sharing needed information during a product's entire "life-cycle". PLM* is one of four corporate ICT "cornerstones", together with SCM, ERP and CRM.

Do I need PDM or PLM? If you still have to ask, the answer is probably PDM. As above, PLM is typically an enterprise-level data and workflow integration between multiple organizations for a product's entire "life cycle". In those cases a need for full-scale PLM is largely self-evident.
*A working draft describing the PLM paradigm is maintained by the International Working Group on PLM.