Professional development is really the individual's responsbility now. Learning is not an event any longer, it's an on-going thing."
-Will Richardson

What is a PLN?
A PLN is a professional learning network. It is a group of networks of different organizations and individuals that help someone to grow professionally and to develop more knowledge in a particular content area. It allows professionals to interact with one another to gather more information and to share new ideas. Social media is a key factor in allowing PLNs to become successful. A PLN allows for professionals around the world to communicate with one another since it is very difficult to meet face-to-face with people who live far away or if they work at a different school. Any questions, comments, or problems can be shared and answered with a PLN.
Why should I create a PLN?
It is valuable to have a PLN to help enrich and enhance teaching. It makes growing professionally easier and a simpler task for teachers to learn on their own. It helps for teachers to complete their own on-going profesional development to help understand how to meet the needs of 21st century learners. Teachers are able to gather resources and grow professionally. Also, teachers can receive peer feedback and collaborate with other professionals. PLNs provide continous learning where professionals can share and network new ideas. Mainly, it encourages teachers to be lifelong learners and engage students in their daily teaching and learning. Since professional developlment is now the teacher's responsbility, by having a PLN, it is manageable to become a lifelong learner and to develop the skills needed to meet the expectations of all learners.

Image from https://twitter.com/advcircle
Additional Information:
Personal Learning Networks for Educators
Professional Development Ideas