CAD PDM, PLM vs ERP*

PDM and PLM do not mean "CAD software". PLM and PDM describe systems for design, manufacturing, change and workflow data management (to which CAD software integrates). Essentially, PDM (Product DATA Management) organizes product design and change files according to a standardized schema of relations between documents and projects - usually for a single organization. PDM software focuses mainly on engineering tasks, i.e. design change, release levels and engineering cycles. PLM (Product LIFECYCLE Management) manages a larger scope than just design file management. PLM enables workflow collaboration between multiple organizations and external partners, integrating with part lists, analysis, and logistics information required during a product's entire "life-cycle". PLM* is one of four corporate ICT "cornerstones", together with SCM, ERP and CRM. Most PLM systems include PDM functionality, but PDM systems don't offer PLM. From another viewpoint, while PDM is all about containment, PLM is more about integration.

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.

PTC ProductPoint is ideal PDM Software for small business. ProductPoint organizes CAD CAM CAE engineering files in a central, easily managed company portal using Microsoft SharePoint Services. More information about PTC/CADDIT SharePoint products can be read HERE.
*A working draft describing the PLM paradigm is maintained by the International Working Group on PLM.