CodePen is a social development environment. At its heart, it allows you to write code in the browser, and see the results of it as you build. A useful and liberating online code editor for developers of any skill, and particularly empowering for people learning to code. We focus primarily on front-end languages like HTML, CSS, JavaScript, and preprocessing syntaxes that turn into those things.
We say social, because CodePen is a community. Most creations on CodePen are public and open source. They are living things that other people and the community can interact with, from a simple hearting, to leaving a comment, to forking and changing for their own needs.
People use CodePen for all sorts of things: prototyping new ideas, reduced test cases for bugs, sending clients things to look at, evaluating potential hires, to finding inspiration. People also use CodePen as a sort of resume and portfolio, showing off their best design and development work.