I'd like others to add to this.
First understand that I'm not a PIM expert. The PIM docs state: PIM is an enterprise data management solution that enables customers to centralize all product information from heterogeneous systems. It allows companies to create a single, enterprise view of their product information, by integrating, standardizing and synchronizing fragmented product data from multiple source (‘Spoke’) systems into a central, operational, data repository (‘Hub’).
PLM deals with the specific data and processes for product development and product quality. The goal of PLM is to reduce product development cost and overall company risk as well as increase revenues by developing better products, faster.
So to be as succinct as I can, PLM is where the day to day work is done for many aspects of product development from ideation to product launch/EOL. It is the authoring tool for the majority of product data that is known at the time of development with specific industry tools used to help in this definition. It also has tools to ensure data completeness, accuracy and cross functional collaboration.
PIM is the repository where the end result of this work is stored so it can then be shared with other systems such as ERP.
PIM consolidates data from many systems where PLM deals with product development data.
It is possible to author this data in PIM, but PLM provides significant functionality specific to product development including project management, R&D and Quality as well as industry specific features.