CircuitPython is a branch of MicroPython created by Adafruit Industries that makes working with microcontrollers simpler. It has excellent support for many of the sensors and components that come with Adafruit devices through its Python libraries. It also allows code to be easily loaded and run without having to install any additional software applications by exposing the microcontroller's storage as a disk drive.
Generally, the differences between MicroPython and CircuitPython are minor, and, in many instances, code will run the same on both implementations.