Moodle comes with a number of core modules that include Activity modules (for example, Quiz and Assignment), Filters (Multimedia plugin or Algebra notation) and Blocks (Calendar) as well as other components such as Enrolments and Authentication plugins, course and grade reports, Repositories and Portfolios, Question types, and so on. While the provided functionality sufficiently satisfies the majority of users, there is growing demand for additional software. Also, requirements change over time and new functionality, for instance, support for certain social networking activities, is needed in your Moodle system.

Due to Moodle's open source nature and modularity (that's what the "M" in Moodle stands for, after all), it is relatively straightforward for developers to add new functionality or modify existing features. These can range from minor modifications (patches) or hacks to full blown modules.

You can get access to the Plugin directory page via the Modules...