This seems to be a great resource for learning about the Commons.
The commons is a new way to express a very old idea—that some forms of wealth belong to all of us, and that these community resources must be actively protected and managed for the good of all.
The commons are the things that we inherit and create jointly, and that will (hopefully) last for generations to come. The commons consists of gifts of nature such as air, oceans and wildlife as well as shared social creations such as libraries, public spaces, scientific research and creative works.
The linked article also contains definitions of some key terminology:
Externality (or illth)
Open source software
Public trust doctrine
Tragedy of the commons
Trust or stakeholder trust