What is the difference between formControlName and FormControl?

I believe you missed an important point: [formGroup] directive in the second example. formControlName is used together with [formGroup] to save your form multiple dot navigations. For example:

<div>
  <input type="text" [formControl]="myForm.controls.firstName"/>
  <input type="text" [formControl]="myForm.controls.lastName"/>
  <input type="text" [formControl]="myForm.controls.email"/>
  <input type="text" [formControl]="myForm.controls.title"/>
</div>

Is equivalent to:

<div [formGroup]="myForm">
  <input type="text" formControlName="firstName"/>
  <input type="text" formControlName="lastName"/>
  <input type="text" formControlName="email"/>
  <input type="text" formControlName="title"/>
</div>

Now imagine nested FormGroups :)

[formControl] assigns a reference to the FormControl instance you created to the FormControlDirective.

formControlName assigns a string for the forms module to look up the control by name.

If you create variables for the controls, you also don't need the . as mentioned by Harry, but I'd also suggest to use [formGroup] instead because with more complicated forms this can become messy.

constructor(fb: FormBuilder) {
    this.fullName = new FormControl('', Validators.required);
    this.gender = new FormControl('');
    this.myForm = fb.group({
        'fullname': this.fullName,
        'gender': this.gender
    });
}