ELK stands for Elasticsearch, Logstash and Kibana.
Brief definitions:
Logstash: It is a tool for managing events and logs. You can use it to collect logs, parse them, and store them for later use (like, for searching). Speaking of searching, logstash comes with a web interface for searching and drilling into all of your logs. It is fully free and fully open source.
Elasticsearch: Elasticsearch is a search server based on Lucene. It provides a distributed, multitenant-capable full-text search engine with a RESTful web interface and schema-free JSON documents.
Kibana: A nifty tool to visualize logs and timestamped data. … Elasticsearch, Logstash and Kibana (ELK Stack)