Do I need app.component.ts?

不必要.
这只是.ts文件的名称.它可以是任何其他组件.但至少需要一个模块和组件来启动Angular2 App.

了解下面的事情,

main.ts

import { platformBrowserDynamic } from '@angular/platform-browser-dynamic';
import { AppModule } from './app.module';   //<<<==== it imports AppModule class from app.module.ts file
const platform = platformBrowserDynamic();
platform.bootstrapModule(AppModule);        //<<<===we bootstarp our AppModule here

app.module.ts // .ts文件的名称

//contents are important as it contains @NgModule({}) decorator.

import { NgModule }      from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { SomeComponent }   from './some.component';  
                                           //<<<===we import SomeComponent class from some.component.ts file to bootstrap it.


@NgModule({
  imports:      [ BrowserModule],declarations: [ SomeComponent ],bootstrap:    [ SomeComponent ]          //<<<===we are going to bootstrap/initialize SomeComponent as our first component.
})
export class AppModule { }                 //<<<====we imported AppModule (contains @NgModule decorator) in main.ts as we are going to bootstrap this class which has @NgModule() decorator.

some.component.ts // .ts文件的名称

import { Component } from '@angular/core';
import {UserService} from '../shared/shared.service';
@Component({
  selector: 'my-app',//<<<===make sure this matches with custom HTML tag used in index.html
  template: `<h1>Anglar2</h1>  
  `
})
export class SomeComponent {}