The ELK stack is an open source toolset for data analysis that includes Logstash, Elasticsearch, and Kibana. Logstash collects and parses data from various sources, Elasticsearch stores and indexes the data for fast searching and analytics, and Kibana visualizes the data. The ELK stack can handle large volumes of time-series data in real-time and provides actionable insights. Commercial plugins are also available for additional functionality like monitoring, security, and support.