This is about as valuable as "Rails is a webserver that responds to some HTTP verbs". Common.js isn't even a part of npm (the correct capitalization) or node.
I never said it was, it's not part of it per se, but it's the 'blessed' module/package system on Node.js (the correct capitalization). Which is really sad, because CommonJS is retarded (a file has to actually be executed in order to get it's require & export characteristics and they might arbitrarily change in runtime). It's sad that Node.js as well as the JS ecosystem in general is fairly hostile towards ES6 modules - basically any current support of ES6 modules is just translating then / dumbing them down back to CJS. Hopefully this'll change in the future as ES6 gets more adopted and CommonJS will be left to rot (a fate it deserves and should've already met ages ago).
NPM is quite basic, it pretty much just downloads tarballs recursively from a webserver according to some version spec. The CommonJS module format is fairly loose (and kind of outlived its usefulnes) and so there's no agreed upon package structure and everyone just does whatever they feel like doing, which leads to a huge overall mess. The whole ecosystem is fragmented and very few things are really agreed-upon.