Tina has a simple Event Bus that lets different parts of the CMS communicate with each other. It's possible to hook into some of these events yourself, as well as use Tina's event bus to send and receive your own events.
Tina's plugin system is used extensively by its own libraries, and serves as the foundation for much of its extensibility.
Toolbar Widgets are another type of plugin. Toolbar widget plugins enable you to include custom React components in Tina's toolbar.