Angular 1

Angular 1 was the first Angular framework to really make single page applications and this better way of manipulating the DOM popular back in the times

Angular 2

Angular 2, back at the time in 2015-2016 was a complete rewrite of Angular 1 which fix many of the problems Angular 1 had though; mostly performance issue.

Angular 3

There is no Angular 3 released and Google decided to move to Angular 4 directly.

Angular 4,5,6

Angular 4, 5 and 6 really are just the latest versions or more recent versions of Angular 2.

Angular

In general, we refer to Angular 2 or higher as just Angular.

AngularJS

Refer to Angular 1 to differentiate with Angular 2+, which is Angular mentioned above.

Angular 6 VS Angular 2

Angular 6 is the latest version and it is not a complete rewrite for every new version. Instead we just have incremental small improvements and indeed the code you’ll learn for Angular 2 is pretty-much the same for Angular 6.