Arduino is an open-source electronics platform based on easy-to-use hardware and software. Arduino boards are able to read inputs - light on a sensor, a finger on a button, or a Twitter message - and turn it into an output - activating a motor, turning on an LED, publishing something online. You can tell your board what to do by sending a set of instructions to the microcontroller on the board. To do so you use the Arduino programming language (based on Wiring), and the Arduino Software (IDE), based on Processing.
Arduino has over a decade of projects and history, so you'll find a lot of existing code that you can use with your Circuit Playground Classic.
Since Circuit Playground is now built into Arduino, it's great for beginners - and you can take advantage of the huge Arduino community.
For experts - Arduino is essentially C/C++ with a built in library of hardware interfaces. You can embed assembly, write ultra-fast code, and twiddle registers.