Angular - whatever (next gen ) ng = angular
ngx is angular plus whatever libraries and versions. it does nothing another there is a command or such to run in npm etc..

'ng' is the typical prefix for AngularJS and any associated packages.

When eventually the Angular project evolved and split to AngularJS (utilizing Javascript) and Angular (which uses Typescript and a different architecture as well as a different versioning standard), there had to be some way to differentiate the two.

Initially, Angular was referred to as 'Angular 2' (ng2), but after a versioning correction to some of it's core modules and implementing semantic versioning, this skipped a version 3, and went straight to version 4 (at the time of writing this, it's at version 8). Here's a blog post updated by the Angular team to cover this.

This then lead to packages adding the prefix 'ngx' to identify anything associated with Angular, and simply ng referring to packages associated with AngularJS (which is still going)

I'll add in references to the above claims in an edit.

EDIT

I found the official branding guidelines from Angular which somewhat shed some light on this.

ng = Angular like original ng-bootstrap

ngx = Angular + x (redefined/modern/new/next-gen) update like ngx-bootstrap

Also, many ng packages are running from the old AngularJS (i.e. 1.x) era. ngx packages are designed mostly post that time for Angular 2+.