This folder contains the frontend & backend code for various extensions & plugins used in Oppia codebase.

• actions: Definitions of domain classes for various user actions on Oppia website.
• answer_summarizers: Model definitions for answer summarizers.
• ckeditor_plugins: Plugins used in CKEditor RTE.
• classifiers: Various common services used for tokenization and prediction.
• interactions: The frontend & backend code for interactions supported on Oppia website.
• objects: The frontend & backend code for various typed editors like boolean editor, filepath editor etc.
• rich_text_components: The frontend & backend code for rich text components supported on Oppia RTE.
• value_generators: The frontend & backend code for value generators used in Oppia.
• visualizations: The frontend & backend code for visualizations used in Oppia.