In business management we often hear about systems supporting the management of the various activities; decision and investments related to the adoption of these tools can be significant, so it should be clear.
Let's define better the various terms and related functions.
ERP - the abbreviation comes from Enterprise Resource Planning; in practice the tool to plan the company's resources. Typically manages, the sales, production, inventory, purchasing, accounting.
An ERP system does not necessarily have to be a single block; It depends on the type of business and how much we are willing to spend and even more to what we are willing to "adapt" our organization to align to the operating flows in the program. A key element, however, is that the data is accessed and stored in a structure that makes them accessible to external applications; For example you could have your ERP product from company X, but you decide to develop the reporting functions in a specific way, perhaps with a web-based application
CRM - Customer relationship management indicates the tool for management of the relationship with customers. Basically allows you to have at hand the key data relating to the customer (effective or potential), contacts, phone calls, tenders and so on. If it is integrated with the ERP data can have very significant summary data (ie .: orders in place, payment, offers, etc. ..)
PLM - Product Lifecycle Management is a tool to keep track of the technical evolution of a product. If properly kept updated and possibly integrated with other systems becomes an important aid to quality management; for example it allows to perform reverse searches on such products use a particular type of paint or a specific component.
There are Open Source or proprietary solutions.
To learn more see their entries in Wikipedia: