"CodePen is a collaborative live coding environment where users can showcase their code, receive feedback and gain inspiration from others."
CodePen is a playground for the front end side of the web. It's all about inspiration, education, and sharing.
Need to build a reduced test case to demonstrate and figure out a bug? CodePen is great for that. Want to show off your latest creation and get feedback from your peers? CodePen is great for that. Want to find an example of a particular design pattern for you project? CodePen is great for that.
CodePen PRO takes things further with features like Collab Mode which allows you to pair program in real time and Professor Mode for allowing a group of students to follow you as you teach code and chat with each other.