NW.js is made by Intel
Overall both are in the same family but they are differences between NW.js and Electron.
Since bringing chromium makes the program very big, there are solutions to bridge between web apps and existing, lighter UI frameworks. Most of the time then, the framework is used to create a bridge between HTML/CSS and the existing frameworks components, modules or UI API.
Since all OSes can render webview, it's possible to ask for one at the OS level by providing a bridge. The problem with this solution might be that if the OS has an outdated webview engine, all modern HTML/CSS/JS solutions might not work?
Except for neutralino, most projects of this type tends to use webview, a C/C++/Go library. Several bindings library for other languages already exist.