A collection of tools, brought together under the conceptual notion of openness, interoperability, and learner control. - George Siemens 

Systems that support learners in

• setting their own learning goals
• managing the content and process of learning and
• communicating with others in the process of learning.

- Mark van Hamelen

PLES are comprised of:

• People
• Content
• Services
• Spaces
• Tools

- Stephen Downes 

"The personal learning environment is more of a conferencing tool than it is a content tool. The focus of a personal learning environment is more on creation and communication than it is consumption and completion. It is best to think of the interfaces facilitated by a personal learning environment as ways to create and manipulate content, as applications rather than resources. In particular, that the various channels created by the PLE enable...a student to form a set of connections with a collection of individuals at any given point."

- Stephen Downes The future of online learning: Ten years on Part II 

Dave Cormier’s EdTechPost: A Collection of PLE Diagrams

Graham Attwell on Personal Learning Environments