The talk delves into event stream processing, a technique used to process data in real-time. It explains the differences between various terms such as event sourcing, complex event processing (CEP), and CQRS event sourcing. The speaker discusses how event stream processing can be used to build scalable, reliable, and maintainable applications. They provide examples from different fields, including Google Analytics, Twitter, Facebook, LinkedIn, and Wikipedia, showcasing how event streams can be used to store data in a way that allows for efficient querying and retrieval of information. The talk also touches on the use of Apache Kafka and Apache Samza as systems for high-throughput reliable event streams, and introduces the concept of full-text search on streams.