PLM has the focus of a given product, and aims to maximize the value delivered by that product by managing all relevant aspects of the life cycle from conception (or some other defined point) through to retirement or disposal. PLM can be thought of as the level of management (planning, organizing, motivating, measuring, controlling) that sits above the management of specific activities within the product life cycle, such as development, production, maintenance, marketing, sales, technical support, evolution, etc., And so PLM sits above project management (as applied to a development project), systems engineering management, etc, both logically and often organizationally.
The IT folks and especially business management software vendors have hijacked the term to mean a software system that keeps track of PLM-related information (cynical view).
ERP is a much more general concept. ERP was originally concerned with planning with the aim of ensuring that an enterprise has the resources it needs, in the right numbers/quantities, with the right characteristics, in the right places, at the right times. The resources may be materials, people, or information, for example.
The IT folks, and especially business management software vendors, have hijacked the term to mean a business management system that integrates multiple facets of the business, including typically planning, manufacturing, sales, and marketing. This view of ERP dominates today.
From these descriptions, it is evident that ERP lacks the characteristic of a product focus, except perhaps for a single-product enterprise. It is also evident that ERP is concerned with the nitty gritty of integrating the elements of structure of an enterprise, physically and logically. The purpose of this integration is to sell software and software support (cynical view) or produce an overall more effective enterprise (enterprise management view in hope).
It is evident that ERP supports PLM, for a product-oriented enterprise, but is not, itself, PLM. Systems engineering management is a management sub-discipline that may interface with and/or utilize some aspects of a PLM and/or a ERP system.