Modules

@NgModule({

declarations: [components, directives, pipes],

imports: [modules],

providers: [service providers],

entryComponents: [components],

exports: [modules, components, directives, pipes],

bootstrap: [AppComponent]

})

takes a metadata object that describes how to

compile a component's template
create an injector at runtime

declarations

declarables are components, directives and pipes
declarables must belong to exactly one module
these declared classes are visible only within the module

providers

you list services here, they are available app-wide
provider is an instruction to the DI system on how to obtain a value for a dependency
most of the time, these dependencies are services that you create and provide
when the Angular router lazy-loads a module, it creates a new injector (child)

bootstrap

you can put more than one component
these components are automaticaly added to entryComponents

entryComponents

loads imperatively, (which means you’re not referencing it in the template), by type
bootstrap and routed components are entryComponents automatically
if a component isn't an entry component and isn't found in a template, the tree shaker will throw it away

A component can also be bootstrapped imperatively:

import { NgModule, ApplicationRef, DoBootstrap } from '@angular/core';
@NgModule({
imports: [ BrowserModule ],
declarations: [ AppComponent ],
entryComponents: [ AppComponent ]
})
export class AppModule implements DoBootstrap {
ngDoBootstrap(appRef: ApplicationRef) {
appRef.bootstrap(AppComponent);
}
}

Feature modules

Domain feature modules (dedicated to a particular application domain like editing a customer)
Routed feature modules (whose top components are the targets of router navigation routes)
Routing modules (defines routes, adds guard and resolver service providers to the module's providers)
Service feature modules (they consist entirely of providers(like HttpClientModule). The root AppModule is the only module that should import service modules)
Widget feature modules (a widget module makes components, directives, and pipes available to external modules)