Backbone models describe how data is stored using JavaScript objects. Each model is a hash of fields called attributes and the behaviour of the model including validation is described by options.

A model of Todo item in a TodoApp would be

var ToDo = Backbone.Model.extend({
  defaults: {
    assignee: '',
    task: ''
  },

  validate: function(attrs) {
    var errors = {},
        hasError = false;

    if(!attrs.assignee) {
      errors.assignee = 'assignee must be set';
      hasError = true;
    }

    if(!attrs.task) {
      errors.task = 'task must be set';
      hasError = true;
    }

    if(hasError) {
      return errors;
    }
  }
});

var Vehicle = Backbone.Model.extend({
  
  description: function () {
    return 'I have ' + this.get('wheels') + ' wheels';
  }
  
});

var Bicycle = Vehicle.extend({
  
  defaults: {
    wheels: 2
  }
  
});

var Car = Vehicle.extend({
  
  defaults: {
    wheels: 4
  }
  
});

var bike = new Bicycle();
bike.description() // I have 2 wheels;

var car = new Car();
car.description() // I have 4 wheels;;

By default, the urlRoot property is not defined. This urlRoot property is used by the url method to create a relative URL where the model's resource would be located on the server.

var User = Backbone.Model.extend({
  
  urlRoot: '/api/users',

  // or

  urlRoot: function () {
    return '/api/users'
  }
  
});

var user = new User();

The url method will firstly check if the model's idAttribute (defaulted at 'id') has been defined. If not, the model isNew and url will simply return the results of urlRoot.

user.url() // /api/users

If the model's idAttribute has been defined, url will return the urlRoot + the model's idAttribute

user.set('id', 1);
user.url() // /api/users/1

Calling save on a new model will result in a POST request to the results of url

var user = new User({ username: 'johngalt' });
user.save() // POST http://webroot/api/users

Calling save on an existing model will result in a PUT request to the results of url

user.set('id', 1);
user.set('username', 'dagnytaggart');
user.save() // PUT http://webroot/api/users/1

Calling fetch on an existing model will result in a GET request to the results of url

user.fetch() // GET http://webroot/api/users/1

Calling destroy on an existing model will result in a DELETE request to the results of url

user.destroy() // DELETE http://webroot/api/users/1