Major release version of Angular are:

• Angular JS

• Angular 2

• Angular 4

• Anugular 5

• Angular 6

The version 2.x and higher is called 'Angular' and the version 1.x or less is called 'Angular JS'

Angular JS:

AngularJS is a pure JavaScript framework, and models in AngularJS are plain old JavaScript objects. AngularJS follows the traditional MVC architecture that comprises a model, a view and a controller

Angular 2:

Angular 2+ introduced TypeScript as the default language for building applications. TypeScript is a syntactic superset of JavaScript that compiles down to plain JavaScript. The Angular team chose TypeScript over JavaScript because of the type annotation feature that lets you do optional static type checking.Every Angular application has at least one component known as the root component. Each component has an associated class that’s responsible for handling the business logic and a template that represents the view layer. Multiple, closely related components can be stacked together to create a module and each module forms a functional unit on its own

Angular 4:

1. Animation Package package out from @angular/core and put into separate package. That mean if you don’t want to use that module , this extra code will not end up in your production bundles.

2. provide support of latest version of typesctipy i.e TypeScript 2.1 & 2.2 compatibility added.

3. make angular bundle Smaller & Faster as compared to previous one

4. ModuleID Removed which we are using for providing relative path during template URL and StyleUrls.

5. <Template> is now <ng-template>

6. Improved *ngIf with else condition.

7. Introduced some new Pipes like titlecase.

Angular 5:(Code is named pentagonal-donut )

Focus was on making Angular smaller, faster, and easier to use. Major changes are:

1. Build Optimizer- production builds created with the CLI will now apply the build optimizer by default

2. Angular Universal State Transfer API and DOM Support - easily share application state between the server side and client side versions of your application

3. Compiler Improvements - Improved the Angular compiler to support incremental compilation

4. TypeScript Transforms - the Angular compiler now operates as a TypeScript transform, making incremental rebuilds dramatically faster.

5. Preserve Whitespace - Historically tabs, newlines, and spaces in your templates have been faithfully recreated and included in your build by the compiler. You can now choose whether or not to preserve whitespace coming from your components and your application

6. Improved Decorator Support

Why there is no Angular 3

Angular is being developed in a MonoRepo (https://github.com/angular/angular) it means a single repo for everything. @angular/core , @angular/compiler , @angular/router etc are in the same repo and may have there own versions. Due to  misalignment of the router package’s version, the team decided to go straight forAngular v4.