What is different between fixture.debugElement.componentInstance and fixture.nativeElement?

In this sample test file I saw two different syntax

one is const app = fixture.debugElement.componentInstance; and another is const compiled = fixture.nativeElement; I don't know what is different between these two syntax?

I am totally new with angular testing and I am applying it to my project but it is confused to me a little bit about this.

describe('AppComponent (initial CLI version)', () => {
  beforeEach(async(() => {
    TestBed.configureTestingModule({
      declarations: [
        AppComponent
      ],
    }).compileComponents();
  }));
  it('should create the app', async(() => {
    const fixture = TestBed.createComponent(AppComponent);
    const app = fixture.debugElement.componentInstance;
    expect(app).toBeTruthy();
  }));
  it(`should have as title 'app'`, async(() => {
    const fixture = TestBed.createComponent(AppComponent);
    const app = fixture.debugElement.componentInstance;
    expect(app.title).toEqual('app');
  }));
  it('should render title in a h1 tag', async(() => {
    const fixture = TestBed.createComponent(AppComponent);
    fixture.detectChanges();
    const compiled = fixture.nativeElement;
    expect(compiled.querySelector('h1').textContent).toContain('Welcome to app!');
  }));
});