Logstash is a server-side data processing pipeline from Elastic. It ingests data from many sources at once, such as logs, metrics, web applications, data stores and cloud services, transforms it, and sends it to one or more destinations. Elasticsearch is the primary output, although many others are supported. Logstash is software that you download and run yourself, centrally managed if desired through Elastic's Pipeline Management UI.
Logstash is developed by Elastic N.V., a company in the Dutch NV legal form headquartered in Amsterdam, with operations also in San Francisco. Code outside the x-pack directory is licensed under Apache 2.0, while x-pack code uses the Elastic License. According to an Elastic staff member in a community forum, builds with an oss suffix are Apache 2.0 only. Because you run it on your own infrastructure, data stays where you deploy it.