According to Wikipedia, one of the simplest places to start, a PLN is:
A personal learning network is an informal learning network that consists of the people a learner interacts with and derives knowledge from in a personal learning environment. In a PLN, a person makes a connection with another person with the specific intent that some type of learning will occur because of that connection.
An important part of this concept is the theory of connectivism developed by George Siemens and Stephen Downes. Learners create connections and develop a network that contributes to their professional development and knowledge. The learner does not have to know these people personally or ever meet them in person.
The following is an excerpt from Dryden’s and Vos’ book on learning networks:
“For the first time in history, we know now how to store virtually all humanity’s most important information and make it available, almost instantly, in almost any form, to almost anyone on earth. We also know how to do that in great new ways so that people can interact with it , and learn from it.”
Then lets look at PLE (Personal Learning Environment)
Personal learning networks share a close association with the concept of personal learning environments. Martindale & Dowdy  describe a PLE as a “manifestation of a learner’s informal learning processes via the Web”.