Some of the key features of MakeCode are:
- web based editor: nothing to install
- cross platform: works in most modern browsers from tiny phone to giant touch screens
- compilation in the browser: the compiler runs in your browser, it's fast and works offline
- works offline: once you've loaded the editor, it stays cached in your browser.
- event based runtime: easily respond to button clicks, shake gestures and more
- makecode.adafruit.com specifically applies to the Adafruit Circuit Playground Express only
- maker.makecode.com aims at supporting the Adafruit Express boards (and more boards from different manufacturers), with an emphasis on breadboarding support.
Yes, Maker is open source under MIT at https://github.com/Microsoft/pxt-maker.