Jetpack is a plugin that provides additional features for self-hosted WordPress sites like social sharing, stats, and comments. It requires a WordPress.com account and connection. Some key features include Publicize for automatic cross-posting, Sharing for social media icons, Jetpack Comments, Subscriptions to follow a site, and WordPress.com Stats for analytics. It also includes extra widgets, contact forms, and a carousel. Installation involves downloading the plugin and connecting to a WordPress.com account.