A Collection can be defined as an ordered set of modules. In Backbone.js, there is a collection class which provides some useful methods to deal with the collections. We can extend the collection class to provide some additional functionalities.

In Backbone.js, a Collection is a structured set of models. It acts as an ordered or unordered list of models that can be sorted, filtered, and manipulated in various ways. Collections are useful for managing groups of related models, such as a list of items fetched from a server or a subset of models based on certain criteria.

Here’s a breakdown of the key characteristics and functions of a Collection in Backbone.js:

Container for Models: A Collection serves as a container for a group of models. These models could represent data fetched from a server, generated locally, or manipulated in the client-side application.
Events Handling: Collections trigger events when models are added, removed, or modified. This allows other parts of the application to respond to changes in the collection.
Fetching and Syncing: Collections provide methods for fetching data from a server and synchronizing it with the client-side application. This involves RESTful API interactions to retrieve, create, update, and delete models.
Iteration and Manipulation: Collections offer methods for iterating over the models they contain, as well as for adding, removing, sorting, and filtering models based on various criteria.
Validation: Collections can enforce validation rules on the models they contain, ensuring that the data remains consistent and valid within the context of the application.
Customization: Collections can be extended and customized to add additional functionality or modify existing behavior to suit the specific requirements of the application.

Overall, Collections in Backbone.js provide a structured way to manage groups of related models, offering powerful features for organizing, manipulating, and synchronizing data within client-side applications.