What are the differences between template driven forms and reactive forms in angular 2 - angular2-forms

Question

In angular 2 what is the difference between the template driven forms and reactive form. In which situations we need to use the template driven forms and reactive forms

Answer 1

Simply we can say
Reactive form can be used in the following situation
Complex forms with more number of fields.
Multiple complex validation are there. Custom validations are required
Require JSON structure to be send with the values in the form.
We can get entire form in a structured way by using "form.value"
If we have 4 fields First Name, Last Name, Email, Phone Number in reactive form.
HTML code will be
<form [formGroup]="form">
First Name <input formControlName="firstName">
Last Name <input formControlName="lastName">
Email <input formControlName="email">
Phone Number <input formControlName="phoneNumber">
</form>
We can get the values from the form like below
{
"firstName": "FName",
"lastName": "LName",
"email": "test#123.com",
"phoneNumber": "123"
}
by calling form.value, where form is FormGroup Variable that we created.
Template Driven Form :
It can be used when using simple forms. Like login page.
With the two way data binding. We can simply assign value to variable from ui and vice versa.
Simple example is if we are givng two way binding for the below input.
<input [(ngModel)]="username">
We can simply display the value that user is giving in the UI.
<p>Hello {{username}}!</p>

Answer 2

A comprehensive answer to a similar question can be found here:
What are the practical differences between template-driven and reactive forms?
here is what Aravind is posted which is abstract and direct to the point. so I copy and paste the entire comparison:
Template Driven Forms Features
Easy to use
Suitable for simple scenarios and fails for complex scenarios
Similar to AngularJS
Two way data binding(using [(NgModel)] syntax)
Minimal component
Automatic track of the form and its data(handled by Angular)
Unit testing is another challenge
Reactive Forms Features
More flexible, but needs a lot of practice
Handles any complex scenarios
No data binding is done (immutable data model preferred by most
developers)
More component code and less HTML markup
Reactive transformations can be made possible such as
Handling a event based on a debounce time
Handling events when the components are distinct until changed
Adding elements dynamically
Easier unit testing
A side by side pros and cons

Answer 3

With the template driven approach you basically apply directives, such as ngModel, in your template. Based on these directives Angular will create formcontrol objects. This approach is good for building simple forms with basic validation (required, minlength, maxlength,...).
With the reactive approach you basically need to create new instances of the formcontrols and formcontrolgroups in your component. Reactive forms are also the best choice for building more complex forms and are better in case you have the intention to implement unit testing for your forms.
Checkout the following topic...
http://blog.angular-university.io/introduction-to-angular-2-forms-template-driven-vs-model-driven/
Angular2 Forms API, Template driven or Reactive?