In AngularJS, we have the concept of modules. Modules there are responsible for grouping pieces of related functionality together and registering it internally during the bootstrap process. Unfortunately, they didn't provide features such as encapsulation and lazy loading.

Angular introduced the NgModules as part of the fifth release candidate of the framework. The main purpose of the new modules is to give a context for the Angular compiler and achieve a good level of encapsulation. For instance, if we are building a library with NgModules, we can have a number of declarations, which are used internally but not exported as part of the public interface. Let's take a look at the following example:

import {NgModule} from '@angular/core';
import {CommonModule} from '@angular/common';
import {TabComponent} from './tab.component';
import {TabItemComponent} from './tab-item.component';

@NgModule({
  imports: [CommonModule],
  declarations: [TabComponent...