ELK stands for Elasticsearch-Logstash-Kibana, a combination of open source products that results in a very popular way to visualize logs in an AWS account:

• Elasticsearch: Based on Apache Lucene, this is a scalable indexing service that is custom built to handle full-text searching. It includes a number of flexible algorithms to help you optimize your search queries.

• Logstash: A project that enables high rates of data ingestion and includes plugins that can handle most of the common log file formats in use by mainstream applications.
• Kibana: A user interface tool that provides a means of visualizing data.

The Amazon Elasticsearch Service is a fully managed implementation of Elasticsearch, with built-in Kibana and supported integrations with Logstash.