Elasticsearch plugins are tools that enhance the core functionality of Elasticsearch by adding features such as security, discovery, and analysis. These plugins fall into two categories: core plugins, maintained by the Elastic team, and community plugins, developed independently. Installing plugins requires specific commands and they must be installed on every node in the cluster, followed by a restart. Core plugins like X-Pack extend Elasticsearch with features like Shield, Watcher, and Marvel, whereas community plugins may include additional configuration steps or integration with third-party tools, like the New Relic plugin for monitoring. Site plugins, which provide graphical interfaces for cluster management, are no longer supported from version 5.x due to security concerns, although workarounds exist. Plugins are essential for filling gaps in Elasticsearch's out-of-the-box capabilities, particularly for production environments, and can be tailored to specific needs, such as using the EC2 Discovery plugin for large AWS deployments.