[angular] What is the difference between declarations, providers, and import in NgModule?

I am trying to understand Angular (sometimes called Angular2+), then I came across @Module:

1. Imports

2. Declarations

3. Providers

Following Angular Quick Start

1. declarations: This property tells about the Components, Directives and Pipes that belong to this module.
2. exports: The subset of declarations that should be visible and usable in the component templates of other NgModules.
3. imports: Other modules whose exported classes are needed by component templates declared in this NgModule.
4. providers: Creators of services that this NgModule contributes to the global collection of services; they become accessible in all parts of the app. (You can also specify providers at the component level, which is often preferred.)
5. bootstrap: The main application view, called the root component, which hosts all other app views. Only the root NgModule should set the bootstrap property.

Adding a quick cheat sheet that may help after the long break with Angular:

DECLARATIONS

Example:

declarations: [AppComponent]

What can we inject here? Components, pipes, directives

IMPORTS

Example:

imports: [BrowserModule, AppRoutingModule]

What can we inject here? other modules

PROVIDERS

Example:

providers: [UserService]

What can we inject here? services

BOOTSTRAP

Example:

bootstrap: [AppComponent]

What can we inject here? the main component that will be generated by this module (top parent node for a component tree)

ENTRY COMPONENTS

Example:

entryComponents: [PopupComponent]

What can we inject here? dynamically generated components (for instance by using ViewContainerRef.createComponent())

EXPORT

Example:

export: [TextDirective, PopupComponent, BrowserModule]

What can we inject here? components, directives, modules or pipes that we would like to have access to them in another module (after importing this module)

Components are declared, Modules are imported, and Services are provided. An example I'm working with:

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';


import { AppComponent } from './app.component';
import {FormsModule} from '@angular/forms';
import { UserComponent } from './components/user/user.component';
import { StateService } from './services/state.service';    

@NgModule({
  declarations: [
    AppComponent,
    UserComponent
  ],
  imports: [
    BrowserModule,
    FormsModule
  ],
  providers: [ StateService ],
  bootstrap: [ AppComponent ]
})
export class AppModule { }

imports are used to import supporting modules like FormsModule, RouterModule, CommonModule, or any other custom-made feature module.

declarations are used to declare components, directives, pipes that belong to the current module. Everyone inside declarations knows each other. For example, if we have a component, say UsernameComponent, which displays a list of the usernames and we also have a pipe, say toupperPipe, which transforms a string to an uppercase letter string. Now If we want to show usernames in uppercase letters in our UsernameComponent then we can use the toupperPipe which we had created before but the question is how UsernameComponent knows that the toupperPipe exists and how it can access and use that. Here come the declarations, we can declare UsernameComponent and toupperPipe.

Providers are used for injecting the services required by components, directives, pipes in the module.

Angular @NgModule constructs:

1. import { x } from 'y';: This is standard typescript syntax (ES2015/ES6 module syntax) for importing code from other files. This is not Angular specific. Also this is technically not part of the module, it is just necessary to get the needed code within scope of this file.
2. imports: [FormsModule]: You import other modules in here. For example we import FormsModule in the example below. Now we can use the functionality which the FormsModule has to offer throughout this module.
3. declarations: [OnlineHeaderComponent, ReCaptcha2Directive]: You put your components, directives, and pipes here. Once declared here you now can use them throughout the whole module. For example we can now use the OnlineHeaderComponent in the AppComponent view (html file). Angular knows where to find this OnlineHeaderComponent because it is declared in the @NgModule.
4. providers: [RegisterService]: Here our services of this specific module are defined. You can use the services in your components by injecting with dependency injection.

Example module:

// Angular
import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { FormsModule } from '@angular/forms';

// Components
import { AppComponent } from './app.component';
import { OfflineHeaderComponent } from './offline/offline-header/offline-header.component';
import { OnlineHeaderComponent } from './online/online-header/online-header.component';

// Services
import { RegisterService } from './services/register.service';

// Directives
import { ReCaptcha2Directive } from './directives/re-captcha2.directive';

@NgModule({
  declarations: [
    OfflineHeaderComponent,,
    OnlineHeaderComponent,
    ReCaptcha2Directive,
    AppComponent
  ],
  imports: [
    BrowserModule,
    FormsModule,
  ],
  providers: [
    RegisterService,
  ],
  entryComponents: [
    ChangePasswordComponent,
    TestamentComponent,
    FriendsListComponent,
    TravelConfirmComponent
  ],
  bootstrap: [AppComponent]
})
export class AppModule { }